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  • Articles  (1,152)
  • Institute of Physics  (620)
  • Blackwell Publishing Ltd  (532)
  • 1995-1999  (853)
  • 1980-1984  (299)
  • 1925-1929
  • 1997  (853)
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  • 1928
  • Medicine  (674)
  • Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics  (478)
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  • Articles  (1,152)
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  • 1995-1999  (853)
  • 1980-1984  (299)
  • 1925-1929
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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of the American Ceramic Society 80 (1997), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: X-ray diffraction (XRD) patterns from nominally β-SiC specimens often differ from those expected for the cubic crystal structure. These differences include the presence of additional peaks, enhanced background intensities, peak broadening, changes in relative peak heights, and shifts in peak positions. It has long been recognized that they are due to the presence of stacking faults, and models relating the experimental observations to stacking fault population have continued to evolve. The presence and relative magnitude of these features vary among different β-SiC specimens. In this work, computer simulations were used to show that the variations are closely related to differences in the type and spatial distribution of stacking faults in each specimen. In these simulations, stacking sequences were generated using a selectively activated 1-D Ising model with a Boltzmann-type probability function for specifying errors, which allows a wide variety of fault configurations to be generated. Direct correlations between different features in the XRD data to the underlying fault population are demonstrated, which are discussed in this paper. It is also shown that this computer model is general, in the sense that many of the models presented in prior work can be interpreted as limiting cases of it.
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  • 2
    ISSN: 1744-313X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Five human teratoma cell lines have been characterized for the presence of a certain number of marker antigens whose presence or absence has been shown to be characteristic of mouse embryonal carcinoma (EC) cells. Four out of the five lines have been shown to respond to at least some of the criteria associated with murine EC cells even though only limited in vitro differentiation could be demonstrated. The significance of certain unusual marker antigen combinations present on the cell line Tera I and its clones and so far unobserved for the murine model is discussed. The observation in Tera I populations of cells carrying simultaneously both the F9 and β2-microglobulin or HLA antigens, suggest that the human cell lines may represent a novel material for the study of mammalian differentiation.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    International journal of immunogenetics 8 (1981), S. 0 
    ISSN: 1744-313X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Principles of Gene Manipulation. Studies in Microbiology
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— This paper examines the application of the Jk, L and M integrals, in complex-variable form, to the Boussinesq wedge. The wedge is symmetrical and subjected to a point couple and point forces at the apex of the wedge. In the case of a point couple acting at the wedge apex the Jy, L and M integrals are found to vanish for all wedge angles whereas Jx displays a 1/r3 path-dependence; where r is a radial dimension measured from the wedge apex. When the wedge is subjected to point forces at the wedge apex then Jx and Jy are 1/r path-dependent whereas L and M are path-independent.The property that the L and M integrals are path-independent for the Boussinesq wedge is applied to the problem of determining the modes I and II stress intensity factors for a corner-loaded edge crack in a half-plane subjected to both normal and parallel point forces to the free surface of the half-plane.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Fatigue tests were performed on thin-walled tubular specimens of S45C steel under tension-compression, pure torsion, in-phase and out-of-phase axial-torsional loadings. The relationship between cracking behaviour and stress components on the crack plane was investigated. Measurement of microcrack density showed that microcracking was governed predominantly by the shear stress amplitude acting on the crack plane for all loading conditions. The failure crack was formed by coalescence of many cracks initiated near the maximum shear planes. The cracks grew turning their orientation to the direction perpendicular to the maximum normal stress. The transition of crack orientation occurred at relatively longer crack lengths at a higher stress ratio. The crack growth behaviour for all loading modes can be correlated using an equivalent strain intensity parameter based on shear and normal strains on the crack plane.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A ductile medium strength steel has been modelled by means of the Gurson model, and been used to investigate the effect of crack tip constraint in several fracture mechanics specimens. Both numerical and experimental results have been obtained, in the course of the crack extension process, for single edge notch bending specimens with different crack length-to-width ratios. The geometries with the shorter cracks always exhibited higher J values at initiation and steeper J crack growth resistance curves, and these results have been explained in terms of the stress and strain fields and damage development in the region ahead of the crack tip.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between-110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A new single-specimen testing method, the normalization method with the so-called LMN calibration function, based on the load separation principle and function calibrations from an individual test record, was used to construct J-R curves directly from load versus load-line displacement records without any additional on-line crack-length monitoring equipment. The research was done on CT-specimens of a glassy polymer PVC at different crosshead speeds ranging from 0.01 to 50 mm/min. The J-R curves evaluated from the normalization method are in good agreement with those from the conventional multiple-specimen testing method in the whole range of the tested crosshead speeds. The results demonstrated the applicability of the normalization method for developing J-R curves at different crosshead speeds in PVC. The crack initiation J-integral values, J0.2, showed a two-regime dependence on the crosshead speeds in the tested crosshead speed range.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The development of fatigue damage in Co45Ni specimens during push—pull and reversed torsion tests, performed inside a scanning electron microscope, was observed and the different stress states compared. It appeared that transgranular crack initiation and development is delayed and intergranular crack initiation promoted under torsional loading. This was explained in terms of reduced surface distortion at the emergence of persistent slip bands (PSBs) and smaller compatibility stresses at the PSB-matrix interfaces. The influence of the mechanical strength of grain boundaries on the difference between tensile and torsional fatigue lives is discussed.
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  • 11
    Electronic Resource
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A Fourier series approach is proposed to calculate stress intensity factors using weight functions for semi-elliptical surface cracks in flat plates subjected to two-dimensional stress distributions. The weight functions were derived from reference stress intensity factors obtained by three-dimensional finite element analyses. The close form weight functions derived are suitable for the calculation of stress intensity factors for semi-elliptical surface cracks in flat plates under two-dimensional stress distributions with the crack aspect ratio in the range of 0.1 ≤a/c≤ 1 and relative depth in the range of 0 ≤a/t≤ 0.8. Solutions were verified using several two-dimensional non-linear stress distributions; the maximum difference being 6%.
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  • 12
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A conventional finite element method may show a weakness when determining the hot spot stress distributions in the brace/chord intersection region of offshore tubular joints. This is because the chosen element displacement functions do not implicitly satisfy the conditions which prevail on the free surfaces. A procedure has been proposed to modify the conventional finite element method so as to allow the hot spot stresses, which occur at the free boundary of the weld toe of tubular joints, to be determined with improved accuracy. The results obtained by this modified method are compared with both an experimental and a traditional finite element solution. The comparison shows that the modified solution is in better agreement with the experimental data as compared with the traditional solution.
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  • 13
    Electronic Resource
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 14
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Simple extensions to the standard deep notch bend test procedure are suggested to allow the collection of data relevant to the energy dissipation rate, D, crack opening angle, COA, and J, all for arbitrarily large amounts of growth in extensive plasticity. The methods of analysis are detailed for real elastic-plastic behaviour of a high strength low-hardening type metal with a view to encouraging use on a wider range of materials. A proposal is made, and equations given, that the particular version of J used for an R-curve derived from the area under the loading diagram, should correspond to the value of the far-field integral, Jff.The relationship between the global measure of COA that emerges from D and the local crack tip opening angle, CTOA, as used in computational studies, is established. Transferability of CTOA data is examined in the light of effects of size and configuration. An explicit rule of the form CTOA √G =f (material and configuration) is proposed for the modelling of ductile growth in finite element studies. It is applied to a set of data in the literature, for the variation of CTOA with size in the deep notch bend test and for the configurations, bending, double edge and centre cracked tension.
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  • 15
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— In this investigation the Electron Channelling Contrast (ECC) technique in scanning electron microscopy (SEM) was applied to reveal the dislocation structures in the vicinity of surface fatigue cracks in comparison to those of cyclically-deformed recrystallized polycrystalline copper. The plastic zone around a fatigue crack was found to consist of an innermost region containing cells, followed by a region containing dense veins and PSBs, surrounded by a structure of loose veins, bundles and loop patches typical of the cyclically deformed matrix. A relation between plastic strain amplitude values deduced from cyclic stress-strain investigations and the dislocation structures near fatigue cracks are given. Typical regions of damage accumulation were identified and plastic strain contours for surface fatigue cracks established. The essentially non-destructive ECC technique is particularly suited to identify the changes in mesoscopic dislocation structures from surface layers to the interior of specimens over large specimen areas.
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  • 16
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 17
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    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The propagation behaviour of fatigue cracks emanating from pre-cracks was numerically simulated to evaluate the development of crack closure with crack growth. The crack opening stress intensity factor at the threshold was approximated as a function of the applied stress and the amount of crack extension. Pre-cracked specimens of a medium-carbon steel with a small surface crack and a single-edge crack were fatigued to investigate experimentally the initiation and propagation of cracks from pre-cracks. Crack closure was dynamically measured by using an interferometric strain/displacement gauge. The threshold condition of crack initiation from pre-cracks was given by a constant value of the effective stress intensity range which was equal to the threshold value for long cracks. The cyclic R-curve was constructed in terms of the threshold value of the maximum stress intensity factor as a function of crack extension approximated on the basis of the experimental and numerical results. The cyclic R-curve method was used to predict the fatigue thresholds of pre-cracked specimens. The predicted values of the fatigue limits for crack initiation and fracture, and the length of non-propagating cracks agreed very well with the experimental results.
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  • 18
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A basic study was performed on the evolution of three-dimensional shapes of small surface fatigue cracks during fatigue, and the effect of this evolution on small-crack growth behavior of a titanium-base alloy. Specifically, the nature and the magnitude of variations in crack aspect ratio, a/c (a is the crack depth and c is the half-surface crack length), during cyclic crack growth and its impact on growth rates have been studied. Experiments were performed on naturally initiated micro-cracks in a microstructure consisting of equiaxed primary-α2 phase in a Widmanstätten (transformed β) matrix. Several cracks under stress ratio (R) levels of 0.1 and −1, were studied. A specialized experimental system, consisting of a laser interferometer (to measure precisely the small-crack surface displacements), and a photo microscope (to automatically and continuously photograph the fatigue micro-cracks) was employed in the study. Apparent aspect ratios of surface cracks were calculated from the compliance response and the surface crack length data as a function of fatigue cycles. These data enabled accurate calculations of growth rates at the surface crack tip as well as the tip at depth in the bulk over the entire crack growth period, thus giving an insight into the crack growth process. Measurements of closure levels of small cracks were also performed and were used to partly account for the differences in growth rates. In the comparisons of small-crack growth data with the large-crack data, surface growth rates correlated relatively well with the large-crack data. Growth rates at depth exhibited large variations due to the irregularity of crack fronts at this location, and these rates deviated significantly from the large-crack behavior. Additionally, these growth rates varied between different cracks. An attempt was made to rationalize these observations in terms of the effects of inhomogeneities present in the microstructure.
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  • 19
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A new technique, known as crack modelling, is used here to predict fatigue failure in a crankshaft component. The technique uses a linear elastic finite element (FE) analysis to derive a stress intensity factor (K) for the component under load. The novel feature of the technique is that K is calculated without introducing a crack into a component; the stress field around the maximum stress point is examined and compared to that for a standard centre-cracked plate. The fatigue limit for a crankshaft was successfully predicted, when compared to experimental data. The only material parameter required for this prediction was the threshold stress intensity range, ΔKth.
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  • 20
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The effects of bluing, associated with drawing strain, on the fatigue strength of eutectoid steel wires have been investigated. The fatigue limit increases by bluing and the increase is more significant with higher drawing strain. The peak in the fatigue limit with regard to the drawing strain in the wires, at a strain of 2.5, disappears after bluing. On the other hand, in the ferritic steel wires investigated for comparison, the fatigue limit gradually increases with the drawing strain up to 7.7. Furthermore, no appreciable change in the fatigue limit due to bluing is found. Based on the results of hardness tests on fatigue specimens with- and without-bluing, it is deduced that the decrease of the fatigue limit beyond the peak drawing strain in the eutectoid steel wire can partly be attributed to insufficient locking of the high-density dislocations by solute atoms. The effect of relaxation of residual stress during bluing is also briefly discussed.
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  • 21
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    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— It is well known that for very short cracks the stress intensity factor K is not a suitable parameter to estimate the stress level over the small but finite Stage II process zone activation region of size rs near the crack tip, within which crack growth events take place. A critical appreciation of the reasons for the limitations on the applicability of ΔK as a fatigue crack propagation (FCP) parameter, when the crack length a is of the same order of magnitude or smaller than the size of the ‘fatigue-fracture activation region’, rs is presented. As an alternative to ΔK the range Δσs of the cyclic normal stress at a point situated at the fixed distance s=rs/2, ahead of the crack tip, inside the fatigue-fracture activation region, is proposed. It is observed that the limitation on the use of ΔK when the crack is short, is mathematical (and not physical) but this inconvenience is easily circumvented if the stress Δσs at the prescribed distance is used instead of ΔK since nowadays Δσs can be obtained numerically by using finite element methods (FEM). It follows that the parameter Δσs is not restricted by the mathematical limitations on ΔK and so it would seem that there is, a priori, no reason why the validity of the parameter Δσs cannot be extended to short cracks. It is shown that if the Paris law is expressed in terms of Δσs (πrrs)½ instead of ΔK the validity of the modified Paris law can be extended to short cracks.A coherent estimate of the value of the fatigue-fracture activation region rs is derived in terms of the fatigue limit ΔσFL obtained from S-N tests and of the threshold value ΔKth obtained from tests on long cracks where both relate to Stage II crack growth that ends in failure, namely, rs= (ΔKth/ΔσFL)2/π. An overall, threshold diagram is presented based on the simple criterion that, for sustained Stage II FCP, Δσs must be greater than ΔσFL. The study is based on a simple continuum mechanics approach and its purpose is the investigation of the suitability of both ΔK and Δσs to characterise the crack driving force that activates complex fracture processes at the microstructure's scale. The investigation pertains to conditions that lead to the ultimate failure of the component at values of Δσs 〉 ΔσFL.
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  • 22
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The boundary value problem for an arbitrarily shaped plane crack embedded in a 3D linear elastic solid can be reduced to a governing hyper-singular integral equation. A discretizing procedure based on a triangulation of the crack area has been offered in Part I of this work. The main goal of Part I is to introduce the analytical results for the 18 resulting finite-part integrals defined over a triangular mesh area. The finite-part integrals occur in those triangles where the source point coincides with one of the element nodes. Mostly the source point lies outside of the considered triangle. In these cases the occurring area integrals are regular.The aim of Part II is, therefore, the derivation of the closed form expressions for the relevant 18 regular area integrals. The resulting relations are of algebraic form which can easily be coded in compact form. Their numerical proof by two different methods shows the highest accuracy and, therefore, the correctness of the final solutions. The relevant numerical results are offered in Appendix I.With the formulae provided in Part I and Part II of the paper the determination of the coefficient matrix, necessary for the calculation of COD values from a linear equation system, is precise and needs only minimum computer time.
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  • 23
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Circumferentially notched cylindrical specimens are tested in torsion to obtain critical J values from crack resistance curves. The specimens are explosion cladded, half ferrite, half austenite, with the interface perpendicular to the cylinder axis and the circumferential notch at, or parallel to, the interface. Critical J values for crack extension in mode III were found to be a factor 1.1 to 2.1 higher than under comparable mode I loading.
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  • 24
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Strength measurements are becoming increasingly important for electroceramics. Bending of specimens small enough to be cut out of small electroceramic components may be one possibility. Therefore the miniaturisation of the 4-point bend-test for ceramic specimens is now being attempted. In this paper the errors in determining the flexural strength arising from the test principle itself, plus the geometry and measuring inaccuracies are calculated and expressed as a function of the outer span length. Contact pressure and a tolerable total measuring inaccuracy determines the dimensions of miniature specimens and fixtures. The possibilities of appropriate specimen preparation are also investigated.Ceramic materials show a volume (i.e. a specimen size) dependence of strength which is described by Weibull's statistical theory. The applicability of the miniature bend-fixtures is demonstrated by measuring this volume effect.
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  • 25
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— This paper describes a versatile technique for simulating the fatigue growth of a wide range of planar cracks of practical significance. Crack growth is predicted on a step-by-step basis from the Paris law using stress intensity factors calculated by the finite element method. The crack front is defined by a cubic spline curve from a set of nodes. Both the 1/4-node crack opening displacement and the three-dimensional J-integral (energy release rate) methods are used to calculate the stress intensity factors. Automatic remeshing of the finite element model to a new position which defines the new crack front enables the crack propagation to be followed. The accuracy and capability of this finite element simulation technique are demonstrated in this paper by the investigation of various problems of both theoretical and practical interest. These include the shape growth trend of an embedded initially penny-shaped defect and an embedded initially elliptical defect in an infinite body, the growth of a semi-elliptical surface crack in a finite thickness plate under tension and bending, the propagation of an internal crack in a round bar and the shape change of an external surface crack in a pressure vessel.
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  • 26
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Cold-expansion of fastener holes is now commonly used within the aerospace industry to increase the fatigue endurance of airframes. Although a number of methods of cold expansion are possible, the split-sleeve cold-expansion process is the most widely accepted and is frequently used in the repair and manufacture stages of both military and civil aircraft. In the present work, the redistribution of residual hoop stresses due to the application of constant amplitude fatigue loading at 4% cold-expanded holes has been studied. A modified Sachs method was adopted to evaluate the residual stress profiles and a replication technique was used to quantify crack growth. It was found that the decay of the residual hoop stress profile near the bore of the hole was due to the initiation and growth of small fatigue cracks. Cracks were found to initiate both near and below the fatigue limit, but subsequently arrested so stabilising the overall residual stress profile.
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  • 27
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The factors affecting the fatigue strength of nitrided titanium were clarified. The fatigue strength depended strongly on the fracture strength of the compound layer formed on the surface by nitriding. We found a Hall-Petch relationship between the fatigue strength of nitrided titanium and the grain size. The findings indicated that the reduction in the fatigue strength by nitriding results from both the formation of the compound layer possessing low fracture strength and grain growth occurring from ordinary nitriding. Furthermore, low-temperature nitriding (620°C, 24 h) was proposed to suppress grain growth. This treatment method improved not only the wear resistance and the corrosion resistance but also the fatigue strength of titanium.
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  • 28
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    Molecular microbiology 24 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The energy requirement for the second step in pullulanase secretion by the general secretory pathway was studied in Escherichia coli. In order to uncouple the two steps in the secretion pathway (across the cytoplasmic and outer membranes, respectively) and to facilitate kinetic analysis of secretion, a variant form of pullulanase lacking its N-terminal fatty acid membrane anchor was used. The transport of the periplasmic secretion intermediate form of this protein across the outer membrane was not inhibited by concentrations of sodium arsenate in excess of those required to reduce ATP levels to ≤10% of their normal value. Pullulanase secretion was inhibited by the protonophore carbonyl cyanide m-chlorophenyl hydrazone at concentrations which were similar to those reported by others to be required to prevent solute uptake or the export and processing of preproteins across the cytoplasmic membrane, but which were in excess of those required to fully dissipate the proton-motive force and to reduce lactose uptake to a significant extent.
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  • 29
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    Molecular microbiology 24 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: The Yersinia pseudotuberculosis pH 6 antigen mediates haemagglutination and adhesion to cultured mammalian cells. The synthesis of pH 6 antigen requires the products of the psaEFABC genes in both Yersinia pseudotuberculosis and Escherichia coli. In-frame deletion mutations of psaE and psaF caused defective haemagglutination. In contrast, we showed that the psaABC genes were sufficient for haemagglutination if they were expressed by a heterologous promoter. Environmental regulation of pH 6 antigen by temperature and pH occurs via regulation of the major pilus protein PsaA at the transcriptional level. Northern blot analyses indicate that the psaA transcript was absent in either psaE or psaF mutant strains. Primer extension analyses indicate that, in Y. pseudotuberculosis, the transcription of the psaE and psaF genes is constitutive. Alkaline phosphatase fusion studies confirm the topology prediction that PsaE and PsaF are both inner-membrane-associated proteins. PsaE consists of an N-terminal cytoplasmic domain, containing sequence similarity to transcriptional regulators found in two-component systems as well as to the Salmonella typhimurium HilA protein, with a C-terminal domain that is periplasmically localized. PsaF is predicted to be oriented with most of the protein in the periplasm, the hydrophobic N-terminus being either integrated in the inner membrane or cleaved as a signal peptide.
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  • 30
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Mycoplasma mycoides contains a signal-recognition particle (SRP) composed of an RNA molecule and an SRP54 homologue (Ffh). We have now identified a mycoplasma homologue to the α subunit of the mammalian SRP receptor and Escherichia coli FtsY. The protein (MmFtsY) was expressed in E. coli and purified to homogeneity. MmFtsY has a weak intrinsic GTPase activity but GTP hydrolysis was markedly stimulated when it was combined with mycoplasma Ffh (MmFfh) and SRP RNA. Also, in the absence of SRP RNA GTPase activity was significantly enhanced. Furthermore, GTP hydrolysis was stimulated when MmFtsY was combined with the N-terminal GTPase domain (N+G) of MmFfh. These findings indicate that basic features of the GTPase activation mechanism are independent of the C-terminal M domain of the MmFfh protein. We propose that the activation is mediated to a large extent by contacts between the GTPase domains of the mycoplasma Ffh and FtsY proteins and that the contribution of the M domain and SRP RNA in the activation mechanism is mainly for modifying the conformation of the MmFfh GTPase domain.
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  • 31
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    Topics: Biology , Medicine
    Notes: The structural genes for the cyanide-insensitive terminal oxidase (CIO) of Pseudomonas aeruginosa were sequenced. The locus comprised two open reading frames, cioA and cioB, coding for gene products of 488 and 335 amino acid residues with predicted molecular masses of 54 241 and 37 016 Da respectively. These genes were encoded by a 2.7 kb transcript and probably comprise an operon. Upstream of a major transcriptional start site is a −10 promoter region and, approximately at nucleotides −50 and +13, there are sequences homologous to the binding site of the transcriptional regulator Anr. The deduced amino acid sequences of CioA and CioB are homologous to the cytochrome bd quinol oxidases of Escherichia coli and Azotobacter vinelandii. However, no cytochrome d-like signals were found in wild-type P. aeruginosa strains. An atypical cytochrome d-like signal was seen under low-aeration growth conditions but only in strains in which the cioAB genes were present on a high-copy-number plasmid. The appearance of these cytochrome d-like signals was not paralleled by a concomitant increase in CIO activity. These data support the hypothesis that the CIO of P. aeruginosa does not contain haem d. This raises the possibility that there is a family of bacterial quinol oxidases related to the cytochrome bd of E. coli that can differ in their haem composition from the E. coli paradigm.
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  • 32
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    Molecular microbiology 24 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Most small multicopy plasmids of Gram-positive bacteria and many in Gram-negative bacteria replicate by a rolling-circle (RC) mechanism. The replication initiator proteins encoded by the RC plasmids and single-stranded bacteriophages of Escherichia coli have origin-specific nicking-closing activities that are required for the initiation and termination of RC replication. We have investigated the sequence requirements for termination of RC replication of plasmid pT181. The initiator nick site is located in the loop of a hairpin region (IRII) within the pT181 origin of replication. By mutational analysis, we have found that several nucleotides within the stem of IRII which are critical for the initiation activity are dispensable for termination of replication. We also demonstrate that nucleotides in the right arm of IRII, but not the left arm, are absolutely required for termination of RC replication. We have also identified specific nucleotides in IRII that are critical for its termination activity. The sequence of the right arm of the hairpin must be located downstream of the initiator nick site for termination, suggesting that termination requires a specific orientation of the initiator protein at the origin.
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  • 33
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    Molecular microbiology 24 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Bacillus subtilis responds to signals of environmental and metabolic stress by inducing over 40 general stress genes under the control of the σB transcription factor. σB activity is regulated post-translationally by a multicomponent network composed of two coupled partner-switching modules, RsbX-RsbS-RsbT and RsbU-RsbV-RsbW, each containing a serine phosphatase (X or U), an antagonist protein (S or V), and a switch protein/serine kinase (T or W). The upstream module (X-S-T) is required to transmit signals of environmental stress. In contrast, the downstream module (U-V-W) is required to transmit signals of energy stress as well as the environmental signals conveyed to it from the upstream module. Until now the function of the rsbR gene product was unknown. RsbR shares significant sequence similarity with the RsbS and RsbV antagonist proteins whose phosphorylation states control key protein–protein interactions within their respective modules. Here we present evidence that RsbR is associated with RsbS in the upstream, environmental-sensing module. To investigate RsbR function, we constructed deletion and point mutations within rsbR and tested their effects on expression of σB-dependent reporter fusions, both singly and in combination with other rsb mutations. To determine the possible interaction of RsbR with other Rsb proteins, we tested the ability of wild-type or mutant RsbR to activate transcription in the yeast two-hybrid system in conjunction with other Rsb regulators. On the basis of this genetic analysis, we conclude that RsbR is a positive regulator which modulates σB activity in response to salt and heat stress. Our data further suggest that: (i) RsbR influences the antagonist function of RsbS by direct protein–protein interaction; and (ii) this interaction with RsbS is likely controlled by the phosphorylation state of RsbR.
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  • 34
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    Molecular microbiology 24 (1997), S. 0 
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Enhancer-dependent transcription in bacteria requires the alternative transcription factor σN (σ54), which forms an RNA polymerase holoenzyme that binds promoters as a transcriptionally inactive complex. We have examined the structure of σN by circular dichroism (CD) analysis. The σN protein and its domains are well structured in the absence of the core RNA polymerase subunits or promoter DNA. Denaturation of σN by temperature as followed by changes in CD shows a concomitant loss of secondary and tertiary structures with a melting temperature of 36°C. The secondary structure displays a two-state melting curve with a second Tm of 85°C. The amino-terminal Region I activation domain together with the acidic Region II does not contribute to the two-state melting. In marked contrast, the integrity of the C-terminal DNA-binding domain is required for the two-state melting. Measurements of pKb also demonstrated that a C-terminal part of σN, but not regions I or I + II, is required for the structural integrity of σN at high pH. Measurements of pKa suggested that α-helical structures are important in σN for the establishment of tertiary structural elements. The tertiary structure near ultraviolet CD signals of σN do not require regions I or I + II but were strongly diminished by C-terminal truncation of σN. Promoter DNA binding resulted in aconformational change in σN, permitting the determination of a binding constant. A typical B-DNA conformation was adopted by the promoter DNA. Implications for the modular domain organization of σN, the function of C-terminal sequences, and domain communication and its role in activation of transcription are discussed.
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  • 35
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    Topics: Biology , Medicine
    Notes: Expression of the global stress protein gene (gspA) is induced during the intracellular infection of macrophages and upon exposure of Legionella pneumophila to in vitro stress stimuli. Transcription of gspA is regulated by two promoters, one of which is regulated by the σ32 heat-shock transcription factor. We utilized a gspA promoter fusion to a promoterless lacZ to probe the phagososmal ‘microenvironment’ for the kinetics of exposure of intracellular L. pneumophila to stress stimuli. Expression through the gspA promoter was constitutively induced by approx. 16-fold throughout the intracellular infection, and occurred predominantly through the σ32-regulated promoter. Expression of the gspA promoter was induced approx. 4.5-fold, 5-, 11- and 9-fold upon exposure of L. pneumophila to heat shock, oxidative stress, acid shock, and osmotic shock, respectively. An isogenic insertion mutant of L. pneumophila in gspA (strain AA224) was constructed by allelic exchange in the wild-type strain AA200. Compared to in vitro-grown wild-type strain AA200, AA224 was more susceptible to all four in vitro stress stimuli. The wild-type phenotypes were restored to strain AA224 by complementation with a plasmid containing wild-type gspA. There was no difference between the wild-type strain and the gspA mutant in cytopathogenicity to U937 cells or in their kinetics of intracellular replication within macrophages and amoebae. However, compared to in vitro-grown bacteria, macrophage-grown and amoebae-grown AA200 and AA224 showed an equal and dramatic increase in resistance to in vitro stress stimuli. Our data showed that regardless of the capacity of L. pneumophila to subvert the microbicidal mechanisms of the macrophage, intracellular L. pneumophila is exposed to a high level of stress stimuli throughout the intracellular infection. Although the GspA protein is required for protection of the bacteria against in vitro stress stimuli, and is induced during intracellular multiplication, the loss of its function is probably compensated for by other macrophage-induced and stress-induced proteins within the intracellular environment.
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  • 36
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: Lysostaphin is an extracellular glycylglycine endopep-tidase produced by Staphylococcus simulans biovar staphylolyticus ATCC1362 that lyses staphylococcal cells by hydrolysing the polyglycine interpeptide bridges of the peptidoglycan. Renewed analysis of the sequence of the lysostaphin gene (Iss), and the sequencing of the amino-terminus of purified prolysostaphin and of mature lysostaphin revealed that lysostaphin is organized as a preproprotein of 493 amino acids (aa), with a signal peptide consisting of 36 aa, a propeptide of 211 aa from which 195 aa are organized in 15 tandem repeats of 13 aa length, and a mature protein of 246 aa. Prolysostaphin is processed in the culture supernatant of S. simulans biovar staphylolyticus by an extracellular cysteine protease. Although prolysostaphin was staphylolytically active, the mature lysostaphin was about 4.5-fold more active. The controlled expression in Staphylococcus carnosus of Iss and Iss with deletions in the prepropeptide region indicated that the tandem repeats of the propeptide are not necessary for protein export or activation of Lss, but keep Lss in a less active state. Intracellular expressed pro- and mature lysostaphin exert staphy-lolytic activity in cell-free extracts, but do not affect growth of the corresponding clones. We characterized a lysostaphin immunity factor gene (lif) which is located in the opposite direction to Iss. The expression of lif in S. carnosus led to an increase in the serine/glycine ratio of the interpeptide bridges of peptidoglycan from 2 to 35%, suggesting that lysostaphin immunity depends on serine incorporation into the interpeptide bridge. If, in addition to lif, Iss is co-expressed the serine/glycine ratio is further increased to 58%, suggesting that Lss selects for optimal serine incorporation. Lif shows similarity to FemA and FemB
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  • 37
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    Topics: Biology , Medicine
    Notes: Catabolite repression of Bacillus subtilis catabolic operons is supposed to occur via a negative regulatory mechanism involving the recognition of a cis-acting catabolite-responsive element (cre) by a complex of CcpA, which is a member of the GalR-LacI family of bacterial regulatory proteins, and the seryl-phos-phorylated form of HPr (P-ser-HPr), as verified by recent studies on catabolite repression of the gnt operon. Analysis of the gnt promoter region by deletions and point mutations revealed that in addition to the ere in the first gene (gntR) of the gnt operon (credown), this operon contains another ere located in the promoter region (creup). A translational gntR-lacZ fusion expressed under the control of various combinations of wild-type and mutant credown and creup was integrated into the chromosomal amyE locus, and then catabolite repression of p-galac-tosidase synthesis in the resultant integrants was examined. The in vivo results implied that catabolite repression exerted by creup was probably independent of catabolite repression exerted by credown; both creup and credown catabolite repression involved CcpA. Catabolite repression exerted by creup was independent of P-ser-HPr, and catabolite repression exerted by credown was partially independent of P-ser-HPr. DNase I footprinting experiments indicated that a complex of CcpA and P-ser-HPr did not recognize creup, in contrast to its specific recognition of credown. However, CcpA complexed with glucose-6-phosphate specifically recognized creup as well as credown, but the physiological significance of this complexing is unknown.
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  • 38
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: A transposition mutant of Bacillus subtilis (designated JC901) that was isolated on the basis of growth inhibition by Na at elevated pH, was deficient in energy-dependent Na extrusion. The capacity of the mutant JC901 for Na -dependent pH homeostasis was unaffected relative to the wild-type strain, as assessed by regulation of cytoplasmic pH after an alkaline shift. The site of transposition was near the 3 -terminal end of a gene, natB, predicted to encode a membrane protein, NatB. NatB possesses six putative membrane-spanning regions at its C-terminus, and exhibits modest sequence similarity to regions of eukaryotic Na+/H+ exchangers. Sequence and Northern blot analyses suggested that natB forms an operon with an upstream gene, natA. The predicted product of natA is a member of the family of ATP-binding proteins that are components of transport systems of the ATP-binding cassette (ABC) or traffic ATPase type. Expression of the lacZ gene that was under control of the promoter for natAB indicated that expression of the operon was induced by ethanol and the protonophore carbonylcyanide p-chlorophenylhydrazone (CCCP), and, more modestly, by Na+, and K+, but not by choline or a high concentration of sucrose. Restoration of the natAB genes, cloned in a recombinant plasmid (pJY1), complemented the Na+-sensitive phe-notype of the mutant JC901 at elevated pH and significantly increased the resistance of the mutant to growth inhibition by ethanol and CCCP at pH 7; ethanol was not excluded, however, from the cells expressing natAB, so ethanol-resistance does not result from NatAB-dependent ethanol efflux. Transformation of the mutant with pJY1 did markedly enhance the capacity for Na+
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  • 39
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: The polA gene of Escherichia coli encodes DNA polymerase I that is involved in DNA replication and repair. Despite the wide knowledge about structure and function of DNA polymerase I, there is little insight into the regulatory mechanisms involved in polA expression. DnaA is the initiator protein for DNA replication in E. coli. There are two putative DnaA-binding sites within the extended promoter region of polA. In this work we studied the influence of altered levels of DnaA protein on polA expression. We found that DnaA overproduction increases polA expression in stationary-phase cultures. The stimulation effect was independent of rpoS, which encodes the sigma factor for stationary-phase-inducible genes. However, it was modulated by ppGpp. Comparative S1 analyses revealed that the induction was based on transcriptional stimulation. Footprint-ing experiments demonstrated that DnaA binds only to the proximal DnaA box near the polA promoter. These results suggest an additional role for DnaA as transcriptional activator of polA at least under certain physiological conditions.
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  • 40
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    Topics: Biology , Medicine
    Notes: Bacillus anthracis, the aetiological agent of anthrax, is a Gram-positive spore-forming bacterium. The cell wall of vegetative cells of B. anthracis is surrounded by an S-layer. An array remained when sap, a gene described as encoding an S-layer component, was deleted. The remaining S-layer component, termed EA1, is chromosomally encoded. The gene encoding EA1 (eag) was obtained on two overlapping fragments in Escherichia coli and shown to be contiguous to the sap gene. The EA1 amino acid sequence, deduced from the eag nucleotide sequence, shows classical S-layer protein features (no cysteine, only 0.1% methionine, 10% lysine, and a weakly acidic pi). Similar to Sap and other Gram-positive surface proteins, EA1 has three 'S-layer-homology’motifs immediately downstream from a signal peptide. Single- and double-disrupted mutants were constructed. EA1 and Sap were co-localized at the cell surface of the wild-type bacilli. However, EA1 was more tightly bound than Sap to the bacteria. Electron microscopy studies and in vivo experiments with the constructed mutants showed that EA1 constitutes the main lattice of the B. anthracis S-layer, and is the major cell-associated antigen.
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  • 41
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    Molecular microbiology 23 (1997), S. 0 
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    Topics: Biology , Medicine
    Notes: Shuttle mutagenesis has been adapted to randomly mutate the genome of Neisseria gonorrhoeae (gono-coccus; Gc). A size-restricted plasmid library of Gc strain FA1090 was mutated with the mini-transposon mTnEGNS. Randomness was tested by checking for transposon insertion bias between vector and insert DNA, Gc transformation efficiency of individual mutated clones, and representation of unique clones before and after Gc transformation with a mutated pool of DNA. Mutants created by random shuttle mutagenesis were screened, using a colony-based polymerase chain reaction assay, for the ability to undergo pilin antigenic variation. Out of 8064 mutants screened, 22 unique transposon insertion mutants were found to be antigenic variation deficient (Avd). The Avd mutants were separated into five types according to recombination defect-associated phenotypes, including colony growth, natural DNA transformation competence, and repair of DNA damage caused by ultraviolet radiation.
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  • 42
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    Molecular microbiology 23 (1997), S. 0 
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    Topics: Biology , Medicine
    Notes: Minichromosomes are plasmids with the origin of chromosome replication, oriC, as their only origin of replication. In Escherichia coli, minichromosomes are compatible with the chromosome and replicate in a cell-cycle-specific manner at the same time as oriC located on the chromosome initiates replication. In int strains, oriC has been inactivated and replaced by a plasmid origin. Because plasmids control their own replication, chromosome replication is uncoupled from the normal cell-cycle control and is random with respect to the cell cycle in the int strains. We have used an intP1 strain to address the question of whether minicromosome replication is coupled to the replication of the chromosome or is governed by cell-cycle-specific signals. Minichromosome replication was analysed by density-shift experiments and found not to be random in the randomly replicating intP1 host. This suggests that the cell-cycle-specific control functions of oriC replication are operating also in the intP1 strain.
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  • 43
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    Notes: The Tar chemotactic signal transducer of Escherichia coli mediates attractant responses to L-aspartate and to maltose. Aspartate binds across the subunit interface of the periplasmic receptor domain of a Tar homodimer. Maltose, in contrast, first binds to the periplasmic maltose-binding protein (MBP), which in its ligand-stabilized closed form then interacts with Tar. Intragenic complementation was used to determine the MBP-binding site on the Tar dimer. Mutations causing certain substitutions at residues Tyr-143, Asn-145, Gly-147, Tyr-149, and Phe-150 of Tar lead to severe defects in maltose chemotaxis, as do certain mutations affecting residues Arg-73, Met-76, Asp-77, and Ser-83. These two sets of mutations defined two complementation groups when the defective proteins were co-expressed at equal levels from compatible plasmids. We conclude that MBP contacts both subunits of the Tar dimer simultaneously and asymmetrically. Mutations affecting Met-75 could not be complemented, suggesting that this residue is important for association of MBP with each subunit of the Tar dimer. When the residues involved in interaction with MBP were mapped onto the crystal structure of the Tar periplasmic domain, they localized to a groove at the membrane-distal apex of the domain and also extended onto one shoulder of the apical region.
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  • 44
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    Notes: In Saccharomyces cerevisiae, two positive transcription factors of the GATA family, Gln3p and NiMp/ Gatlp, upregulate the expression of multiple nitrogen pathway genes via upstream 5-GATA-3′ sequences. Another GATA factor, Uga43p/Da180p, downregulates to varying degrees the expression of some nitrogen-regulated genes. Here, we report the functional analysis of a fourth GATA factor, Gzf 3p/Ni12p, whose gene was discovered by systematic sequencing of chromosome X. The Gzf3 protein most closely resembles Uga43p. Similar to Uga43p, Gzf3p has the properties of a negative GATA factor. While Uga43p is active specifically under nitrogen-derepression conditions, Gzf 3p exerts its negative regulatory function specifically on preferred nitrogen sources: it is involved in nitrogen repression of NiMp-dependent transcription. At least one positive GATA factor is required for the UGA43 and GZF3 genes to be expressed. The Uga43p factor negatively regulates GZF3 expression and vice versa. In addition, both Uga43p and Gzf3p moderately regulate expression of their own genes. These two proteins seem to be parts of a complex network of GATA factors which probably play a determining role in nitrogen-regulated transcription.
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  • 45
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: Formation of araB-lacZ coding-sequence fusions is a key adaptive mutation system. Eighty-four independent araB-lacZ fusions were sequenced. All fusions carried rearranged MuR linker sequences between the araB and lacZ domains indicating that they arose from the standard intermediate of the well-characterized Mu DNA rearrangement process, the strand transfer complex (STC). Five non-standard araB-lacZ fusions isolated after indirect sib selection had novel structures containing back-to-back inverted MuR linkers. The observation that different isolation procedures gave rise to standard and non-standard fusions indicates that cellular physiology can influence late steps in the multi-step biochemical sequence leading to araB-lacZ fusions. Each araB-lacZ fusion contained two novel DNA junctions. The MuR-lacZ junctions showed‘hot-spotting’according to established rules for Mu target selection. The araB-MuR and MuR-MuR junctions all involved exchanges at regions of short sequence homology. More extensive homology between MuR and araB sequences indicates potential STC isomerization into a resolvable four-way structure analogous to a Holliday junction. These results highlight the molecular complexity of araB-lacZ fusion formation, which may be thought of as a multi-step cell biological process rather than a unitary biochemical reaction.
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  • 46
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: Mutations in the seven clustered rpf genes cause downregulated synthesis of extracellular enzymes and reduced virulence of Xanthomonas campestris pathovar campestris (Xcc). The phenotype of mutants in one of the genes, rpfF, can be restored by a diffusible extracellular factor (DSF) produced by all Xcc strains tested, apart from rpfF and rpfB mutants. DSF accumulates in early stationary phase (when synthesis of enzymes is maximal), but levels decline subsequently. Addition of DSF to exponentially-growing wild-type bacteria does not cause precocious enzyme synthesis. rpfB and rpfF are expressed throughout growth, but the rate increases in early stationary phase. RpfB is predicted to be a long-chain fatty acyl CoA ligase, and RpfF shows some relatedness to enoyl CoA hydratases. The properties of DSF suggest that it may be a fatty-acid derivative, and certain lipid preparations possess DSF activity at higher concentrations. These include lipid extracts and acid-hydrolysed lipopolysaccharide and lipid A from Xcc, and purified dodecanoic and hydroxydodecanoic acid. DSF production is confined to certain xanthomonads. We propose a model for the DSF system, which represents a novel mechanism for regulating virulence factor synthesis in response to physiological or environmental changes.
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    Notes: The general amino acid permease, Gap1, of Saccharomyces cerevisiae is very active in cells grown on proline as the sole nitrogen source. Adding NH4+ to the medium triggers inactivation and degradation of the permease via a regulatory process involving Npi1p/Rsp5p, a ubiquitin–protein ligase. In this study, we describe several mutations affecting the C-terminal region of Gap1p that render the permease resistant to NH4+-induced inactivation. An in vivo isolated mutation (gap1pgr ) causes a single Glu→Lys substitution in an amino acid context similar to the DXKSS sequence involved in ubiquitination and endocytosis of the yeast α-factor receptor, Ste2p. Another replacement, substitution of two alanines for a di-leucine motif, likewise protects the Gap1 permease against NH4+-induced inactivation. In mammalian cells, such a motif is involved in the internalization of several cell-surface proteins. These data provide the first indication that a di-leucine motif influences the function of a plasma membrane protein in yeast. Mutagenesis of a putative phosphorylation site upstream from the di-leucine motif altered neither the activity nor the regulation of the permease. In contrast, deletion of the last eleven amino acids of Gap1p, a region conserved in other amino acid permeases, conferred resistance to NH4+ inactivation. Although the C-terminal region of Gap1p plays an important role in nitrogen control of activity, it was not sufficient to confer this regulation to two NH4+-insensitive permeases, namely the arginine (Can1p) and uracil (Fur4p) permeases.
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  • 48
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    Notes: The hlyX gene of the pig pathogen Actinobacillus pleuropneumoniae encodes HlyX, a homologue of FNR, the anaerobic transcription regulator of Escherichia coli. The hlyX gene complements the anaerobic respiratory deficiencies of E. coli fnr mutants but also induces the expression of an otherwise latent haemolysin. Therefore, FNR and HlyX have distinct but overlapping regulons. The hlyX gene has been overexpressed as a gst ::hlyX fusion and the HlyX protein purified. Similar to FNR, HlyX can acquire a [4Fe–4S] cluster, which promotes binding to the FNR box (Kd of 20–30 nM) under anaerobic conditions. Expression of hlyX in E. coli induced the anaerobic production of at least five polypeptides, including the yfiD gene product, which were not induced by fnr. Analysis of the yfiD promoter region revealed the presence of two FNR boxes situated at −61.5 and −114.5. Consistent with this observation, expression from the semi-synthetic Class I promoter FF+20pmelR was efficiently activated by HlyX but not by FNR. The weaker level of FNR-mediated activation of Class I promoters suggests that there is a poorer activating contact (activating region 1 (AR1) equivalent) between FNR and RNA polymerase at these promoters and that HlyX possesses an additional or improved AR1. The AR1 of HlyX is partially characterized by a surface-exposed region around amino acid A187, which confers the altered specificity and provides an explanation for the existence of distinct but overlapping HlyX and FNR regulons.
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  • 49
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    Molecular microbiology 23 (1997), S. 0 
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    Notes: The XylR protein encoded by pWWO, the TOL (toluene biodegradation) plasmid of Pseudomonas putida, activates at a distance the transcription of Pu and Ps, which are the two σ54-dependent promoters of the plasmid, but it also downregulates its own σ70-promoter, Pr, which divergently overlaps the upstream activating sites of Ps. All regulatory elements that control Pr activity have been faithfully reproduced in Escherichia coli, and the basis of the autoregulation of XylR transcription has been examined by monitoring the activity in vivo of different combinations of mutant proteins and promoters in rpoN+ and rpoN- genetic backgrounds. By using PsIPr regions bearing deleted or offset binding sites for XylR and the σ54-containing RNA polymerase, we could show that formation of a nucleoprotein complex involving the polymerase bound to the divergent promoter Ps is not required for downregulation of Pr. Mutant XylR proteins, G268N and A311V (mutated within the NTP-binding region of XylR) or R453H (affected in multi-merization), which are unable to activate (-dependent transcription from Ps, were indistinguishable from the wild-type XylR in their ability to repress a reporter Pr-lacZ fusion. Autoregulation of XylR is therefore due exclusively to the binding of the protein to its target sites at the Pr promoter. This allows one to define sensu stricto XylR as a transcriptional repressor, independently of its activator role in other promoters.
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  • 50
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: Virulence genes of pathogenic bacteria, which code for toxins, adhesins, invasins or other virulence factors, may be located on transmissible genetic elements such as transposons, plasmids or bacteriophages. In addition, such genes may be part of particular regions on the bacterial chromosome, termed‘pathogenicity islands’(Pais). Pathogenicity islands are found in Gram-negative as well as in Gram-positive bacteria. They are present in the genome of pathogenic strains of a given species but absent or only rarely present in those of non-pathogenic variants of the same or related species. They comprise large DNA regions (up to 200 kb of DNA) and often carry more than one virulence gene, the G+C contents of which often differ from those of the remaining bacterial genome. In most cases, Pais are flanked by specific DNA sequences, such as direct repeats or insertion sequence (IS) elements. In addition, Pais of certain bacteria (e.g. uropathogenic Escherichia coli, Yersinia spp., Helicobacter pylori) have the tendency to delete with high frequencies or may undergo duplications and amplifications. Pais are often associated with tRNA loci, which may represent target sites for the chromosomal integration of these elements. Bacteriophage attachment sites and cryptic genes on Pais, which are homologous to phage integrase genes, plasmid origins of replication or IS elements, indicate that these particular genetic elements were previously able to spread among bacterial populations by horizontal gene transfer, a process known to contribute to microbial evolution.
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  • 51
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    Molecular microbiology 23 (1997), S. 0 
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    Notes: The expression of dnaA is autoregulated, in that transcription of the gene increases when DnaA is inactivated (and initiation of replication prevented) and decreases when DnaA is supplied in excess. However, the inactivation of DnaA does not necessarily lead to increased DnaA production, as dnaA(7s; temperature sensitive) strains which are integratively suppressed by derivatives of the plasmid R1 do not show temperature-induced derepression. Several possible explanations for this unanticipated behaviour were considered and ruled out. We suggest here that the completion of a critical step in initiation may prevent dnaA derepression: although DnaA would be required to complete this step at oriC, DnaA(Ts) would be sufficient at the R1 origin. Autoregulation of dnaA has been attributed to the binding of DnaA at a consensus binding site in the dnaA promoter region. We show here, using reporter systems, that this DnaA-binding site is not required for the autoregu-latory response. We find, further, that replacement of the chromosomal dnaA gene with one containing a mutated binding site causes no demonstrable pheno-typic change: cells with the mutant gene show no disadvantage in competition with dnaA+ cells.
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  • 52
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Notes: Many non-lantibiotic bacteriocins of lactic acid bacteria are produced as precursors which have N-terminal leader peptides that share similarities in amino acid sequence and contain a conserved processing site of two glycine residues in positions -1 and -2. A dedicated ATP-binding cassette (ABC) transporter is responsible for the proteolytic cleavage of the leader peptides and subsequent translocation of the bacteriocins across the cytoplasmic membrane. To investigate the role that these leader peptides play in the recognition of the precursor by the ABC transporters, the leader peptides of leucocin A, lactococcin A or colicin V were fused to divergicin A, a bacteriocin from Carnobacterlum divergens that is secreted via the cell's general secretion pathway. Production of divergicin was monitored when these fusion constructs were introduced into Leuconostoc gelidum, Lactococcus lactis and Escherichia coli, which carry the secretion apparatus for leucocin A, lactococcins A and B, and colicin V, respectively. The different leader peptides directed the production of divergicin in the homologous hosts. In some cases production of divergicin was also observed when the leader peptides were used in heterologous hosts. For ABC-transporter-dependent secretion in E. coli the outer membrane protein TolC was required. Using this strategy, colicin V was produced in L. lactis by fusing this bacteriocin behind the leader peptide of leucocin A.
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  • 53
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    Molecular microbiology 23 (1997), S. 0 
    ISSN: 1365-2958
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    Topics: Biology , Medicine
    Notes: Ribonuclease E (RNase E), which is encoded by an essential Escherichia coli gene known variously as rne, ams, and hmp, was discovered initially as an rRNA-processing enzyme but is now known to have a general role in RNA decay. Multiple functions, including the ability to cleave RNA endonucleolyticaliy in AU-rich single-strand regions, RNA-binding capabilities, and the ability to interact with polynucleotide phosphorylase and other proteins implicated in the processing and degradation of RNA, are encoded by its 1061 amino acid residues. The presence of homologues and functional analogues of the rne gene in a variety of prokaryotic and eukaryotic species suggests that its functions have been highly conserved during evolution. While much has been learned in recent years about the structure and functions of RNase E, there is continuing mystery about possible additional activities and molecular interactions of this enzyme.
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  • 54
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    Notes: We report the purification and characterization of the enzyme nucleoside diphosphate kinase (Ndk) from Mycobacterium smegmatis. The N-terminus of the enzyme was blocked but an internal sequence showed approx. 70% homology with the same enzymes from Pseudomonas aeruginosa and Escherichia coli. Immobilization of the mycobacterial nucleoside diphosphate kinase on a Sepharose 4B matrix and passing the total cell extract through it revealed four proteins (P70, P65, P60, and P50, respectively) of Mr 70 kDa, 65 kDa, 60 kDa and 50 kDa that were retained by the column. While the proteins of Mr 70 kDa and 50 kDa modulated the activity of Ndk directing it towards GTP synthesis, the 60 kDa protein channelled the specificity of Ndk entirely towards CTP synthesis. The 65 kDa protein modulated the specificity of Ndk directing it entirely towards UTP synthesis. The specificity for such mycobacterial proteins towards NTP synthesis is retained when they are complexed with P. aeruginosa Ndk. We further demonstrate that the P70 protein is pyruvate kinase and that each of the four proteins forms a complex with Ndk and alters its substrate specificity. Given the ubiquitous nature of Ndk in the living cell and its role in maintaining correct ratios of intracellular nucleoside triphosphates, the implications of the occurrence of these complexes have been discussed in relation to the precursor pool for cell wall biosynthesis as well as RNA/DNA synthesis.
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  • 55
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    Notes: Related outer membrane proteins, termed secretins, participate in the secretion of macromolecules across the outer membrane of many Gram-negative bacteria. In the pullulanase-secretion system, PulS, an outer membrane-associated lipoprotein, is required both for the integrity and the proper outer membrane localization of the PulD secretin. Here we show that the PulS-binding site is located within the C-terminal 65 residues of PulD. Addition of this domain to the filamentous phage secretin, pIV, or to the unrelated maltose-binding protein rendered both proteins dependent on PulS for stability. A chimeric protein composed of bacteriophage f1 pIV and the C-terminal domain of PulD required properly localized PulS to support phage assembly. An in vivo complex formed between the pIV-PulD65 chimera and PulS was detected by co-immunoprecipitation and by affinity chromatography.
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  • 56
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    Molecular microbiology 24 (1997), S. 0 
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  • 57
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    Notes: The closely related B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) both bind strongly to GM1 ganglioside receptors but LTB can also bind to additional glycolipids and glycoproteins. A number of mutant CT B-subunits were generated by substituting CTB amino acids with those at the corresponding positions in LTB. These were used to investigate the influence of specific residues on receptor-binding specificity. A mutated CTB protein containing the first 25 residues of LTB in combination with LTB residues at positions 94 and 95, bound to the same extent as native LTB to both delipidized rabbit intestinal cell membranes, complex glycosphingolipids (polyglycosylceramides) and neolactotetraosylceramide, but not to non-GM1 intestinal glycosphingolipids. In contrast, when LTB amino acid substitutions in the 1–25 region were combined with those in the 75–83 region, a binding as strong as that of LTB to intestinal glycosphingolipids was observed. In addition, a mutant LTB with a single Gly-33→Asp substitution that completely lacked affinity for both GM1 and non-GM1 glycosphingolipids could still bind to receptors in the intestinal cell membranes and to polyglycosylceramides. We conclude that the extra, non-GM1 receptors for LTB consist of both sialylated and non-sialylated glycoconjugates, and that the binding to either class of receptors is influenced by different amino acid residues within the protein.
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  • 58
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    Molecular microbiology 24 (1997), S. 0 
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  • 59
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    Notes: Mycobacteria have the ability to persist within host phagocytes, and their success as intracellular pathogens is thought to be related to the ability to modify their intracellular environment. After entry into phagocytes, mycobacteria-containing phagosomes acquire markers for the endosomal pathway, but do not fuse with lysosomes. The molecular machinery that is involved in the entry and survival of mycobacteria in host cells is poorly characterized. Here we describe the use of organelle electrophoresis to study the uptake of Mycobacterium bovis bacille Calmette Guerin (BCG) into murine macrophages. We demonstrate that live, but not dead, mycobacteria occupy a phagosome that can be physically separated from endosomal/lysosomal compartments. Biochemical analysis of purified mycobacterial phagosomes revealed the absence of endosomal/lysosomal markers LAMP-1 and β-hexosaminidase. Combining subcellular fractionation with two-dimensional gel electrophoresis, we found that a set of host proteins was present in phagosomes that were absent from endosomal/lysosomal compartments. The residence of mycobacteria in compartments outside the endosomal/lysosomal system may explain their persistence inside host cells and their sequestration from immune recognition. Furthermore, the approach described here may contribute to an improved understanding of the molecular mechanisms that determine the intracellular fate of mycobacteria during infection.
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  • 60
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    Notes: The bgl operon of Escherichia coli is rendered cryptic and uninducible in wild-type cells by the presence of DNA structural elements that negatively regulate transcription. We have carried out a detailed analysis of the sequences implicated in negative regulation. Fine-structure deletion analysis of the upstream sequences showed the presence of at least two elements involved in silencing the promoter. Chemical probing of genomic DNA in vivo showed that a region of dyad symmetry, present upstream of the promoter, is hypersensitive to KMnO4. The hypersensitive region detected corresponds to the potential cruciform structure implicated earlier in negative regulation. Enhancement of transcription from the wild-type promoter, observed in the presence of the gyrase inhibitor novobiocin, was absent in a mutant that carried point mutations in the inverted repeat. This observation suggests that the activation seen in a gyrase mutant is mediated by destabilization of the cruciform because of reduced supercoiling. Deletion of sequences downstream of the potential cruciform also resulted in an increase in transcription, indicating the presence of a second regulatory element. Measurement of transcription from the bgl promoter carrying the deletion, in a strain that has a mutation in the hns gene, indicated that this region is likely to be involved in binding to H-NS or a protein regulated by H-NS, which acts as a non-specific repressor. We also provide evidence which suggests that transcriptional activation by mutations at the cAMP receptor protein (CRP)-binding site is mediated partly by antagonization of the negative effect of H-NS by CRP–cAMP as a result of its increased affinity for the mutant site.
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  • 61
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    Molecular microbiology 24 (1997), S. 0 
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  • 62
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    Molecular microbiology 24 (1997), S. 0 
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    Notes: The adjacent, divergently transcribed glpACB and glpTQ operons of Escherichia coli encode the anaerobic glycerol 3-phosphate dehydrogenase and glycerol 3-phosphate transporter/phosphodiesterase, respectively. These operons are negatively controlled by glp repressor binding to operators that overlap the glpA promoter elements. Using DNase I footprinting, three additional operators (OT1–3) were identified at positions +307 to +359 within the glpT coding region. To assess a potential regulatory role for these remote operators in vivo, a glpT–lacZ transcriptional fusion containing all of the glpA and glpT operators was constructed. The response of this fusion to the glp repressor was compared to fusion constructs in which OT1 and OT3 were inactivated, either by deletion or by site-directed mutagenesis. It was found that repression of glpT conferred by binding of glp repressor to glpA operators was increased about three- to fourfold upon introduction of the remote glpT operators. In addition, two integration host factor (IHF) binding sites were identified downstream of the glpT transcriptional start site at positions +15 to +51 and +193 to +227. A regulatory role for IHF was demonstrated by showing that repression of glpT mediated by GlpR was decreased about twofold in strains deficient in IHF and that mutations in IHF1 and/or IHF2 decreased repression about two- to threefold. The effect of IHF was apparent only when the remote operators were present. All of the results are consistent with a model of repression involving GlpR binding simultaneously to the glpA and remote glpT operators, with intervening DNA forming a loop.
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  • 63
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    Molecular microbiology 24 (1997), S. 0 
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    Notes: Newly synthesized polypeptide chains are released from peptidyl-tRNA when the ribosome encounters a stop signal on mRNA. Extra-ribosomal proteins (release factors) play an essential role in this process. Although the termination process was first discovered in the late 1960s, much of the mechanism has remained obscure. However, important steps have recently been made in both prokaryotic and eukaryotic organisms in unlocking the secrets of this vital stage in protein synthesis. In this review we summarize these advances and focus attention on the remaining areas of uncertainty, particularly with respect to the models that have been proposed for the action of the GTP-hydrolysing termination factors in prokaryotes and eukaryotes, i.e. RF3 and eRF3.
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  • 64
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    Molecular microbiology 24 (1997), S. 0 
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    Notes: The σS level in starving (stationary phase) Escherichia coli cells increases four- to sixfold following growth in a defined or a complex medium. Chemostat-grown cells, subjected to increasing carbon starvation, also become progressively richer in σS content. These increases occur despite reduced transcription of the σS-encoding gene, rpoS, and translation of rpoS mRNA, and result solely from a large increase in the stability of the sigma protein. Previous results, based on rpoS ::lacZ transcriptional and translational fusions, and on methionine incorporation in σS, had suggested increased synthesis of σS in starving cells. Alternative explanations for these results consistent with the conclusions of this paper are discussed.
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  • 65
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    Notes: The two major virulence factors of Bacillus anthracis are the tripartite toxin and the polyglutamate capsule, which are encoded by genes on the large plasmids, pX01 and pX02, respectively. The genes atxA, located on pX01, and acpA, located on pX02, encode positive frans-acting proteins that are involved in bicarbonate-mediated regulation of toxin and capsule production, respectively. A derivative strain cured of pX01 produced less capsular substance than the parent strain harbouring both pX01 and pX02, and electroporation of the strain cured of pX01 with a plasmid containing the cloned atxA gene resulted in an increased level of capsule production. An acpA-null mutant was complemented by not only acpA but also the atxA gene. The cap region, which is essential for encapsulation, contains three genes capB, capC, and cap A, arranged in that order. The atxA gene stimulated capsule synthesis from the cloned cap region. Transcriptional analysis of cap by RNA slot-blot hybridization and primer-extension analysis revealed that atxA activated expression of cap in trans at the transcriptional level. These results indicate that cross-talk occurs, in which the pX01-located gene, atxA, activates transcription of the cap region genes located on pX02. We identified two major apparent transcriptional start sites, designated P1 and P2, located at positions 731 bp and 625 bp, respectively, upstream of the translation-initiation codon of capB. Transcription initiated from P1 and P2 was activated by both atxA and acpA, and activation appeared to be stimulated by bicarbonate. Deletion analysis of the upstream region of the cap
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  • 66
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    Notes: In order to address the dynamics of DNA topology in hyperthermophilic archaea, we analysed the topological state of several plasmids recently discovered in Thermococcales and Sulfolobales. All of these plasmids were from relaxed to highly positively super-coiled in vitro, i.e. they exhibited a significant linking excess compared to the negatively supercoiled plasmids from mesophilic organisms (both Archaea and Bacteria). In the two archaeai orders, plasmid linking number (Lk) decreased as growth temperature was lowered from its optimal value, i.e. positively super-coiled plasmids were relaxed whereas relaxed plasmids became negatively supercoiled. Growth temperatures above the optimum correlated with higher positive supercoiling in Sulfolobales (Lk increase) but with relaxation of positive supercoils in Thermococcus sp. GE31. The topological variation of plasmid DNA isolated from cells at different growth phases were found to be species specific in both archaeai orders. In contrast, the direction of topological variation under temperature stress was the same, i.e. a heat shock correlated with an increase in plasmid positive supercoiling, whilst a cold shock induced negative supercoiling. The kinetics of these effects were analysed in Sulfolobales. In both temperature upshift (from 80 to 85C) and downshift (from 80 to 65C), a transient sharp variation of Lk occurred first, and then DNA supercoiling progressively reached levels typical of steady-state growth at the final temperature. These results indicate that DNA topology can change with physiological states and environmental modifications in hyperthermophilic archaea.
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  • 67
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    Molecular microbiology 23 (1997), S. 0 
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    Notes: The fdxN element, along with two other DNA elements, is excised from the chromosome during heterocyst differentiation in Anabaena sp. strain PCC 7120. Previous work showed that rearrangement of the fdxN element requires the xisF gene, which encodes a site-specific recombinase, and suggested that at least one other heterocyst-specific factor is involved. Here we report that the xisH and xisl genes are necessary for the heterocyst-specific excision of the fdxN element. Deletion of a 3.2 kb region downstream of the xisF gene blocked the fdxN-element rearrangement in hetero-cysts. The 3.2 kb deletion was complemented by the two overlapping genes xisH and xisl. Interestingly, extra copies of xlsHI on a replicating plasmid resulted in the xisF-dependent excision of the fdxN element in vegetative cells. Therefore, xisHI are involved in the control of cell-type specificity of the fdxN rearrangement. The xisHI genes had no effect on the two other DNA rearrangements. The xisHl-induced excision of the fdxN element produced strains lacking the element and demonstrates that the 55 kb element contains no essential genes. xisH and xisl do not show similarity to any known genes.
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  • 68
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    Molecular microbiology 23 (1997), S. 0 
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    Notes: Cyanobacteria acclimate to low-temperature conditions by desaturating their membrane lipids. The desB (ω3 desaturase) and desC (A9 desaturase) genes of Synechococcus sp. strain PCC 7002 were cloned and characterized, and the expression of the desA (Δ12 desaturase), desB and desC genes was studied as a function of temperature. The steady-state mRNA abundance for the desA gene was threefold higher in cells grown at 22 C than in cells grown at 38°C. des B transcripts were not detected at 38°C, but were abundant in cells grown at 22°C. Levels of desC mRNA were similar at both growth temperatures. The mRNA levels of each desaturase gene increased within 15min of a temperature shift-down to 22°C, and mRNA levels recovered within 15min after a shift-up to 38°C. The cold-induced accumulation of transcripts from the desA and desB genes was suppressed by the addition of chloramphenicol, but the transient elevation of the desC transcript levels at 22°C was not affected by chloramphenicol. The half-lives of the desA and desB mRNAs were significantly longer in cells grown at 22°C than in cells grown at 38°C, but the desC mRNA had a similar half-life at both temperatures. These studies reveal three patterns of temperature regulation for the desaturase genes, whose expression is tightly controlled by a combination of mRNA synthesis and stabilization. These studies demonstrate that elevation of desaturase mRNA levels is not the rate-limiting event during the low-temperature acclimation of cyanobacteria.
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  • 69
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    Molecular microbiology 23 (1997), S. 0 
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    Notes: The Bacillus subtilis hbs gene encodes an essential chromatin-associated protein termed Hbsu. Hbsu, the counterpart of the Escherichia coli HU protein, binds DNA in a non-specific way but has a clear preference for bent, kinked or altered DNA sequences. To investigate the role of Hbsu in DNA repair and DNA recombination we have constructed a series of site-directed mutants in the hbs gene and used these mutant genes to substitute the wild-type chromosomal hbs gene. The hbs47 mutation, which codes for a mutant protein in which residue Phe-47 has been replaced by Trp, does not cause any discernible phenotype. Additional substitution of residue Arg-55 by Ala (hbs4755 mutation) rendered cells deficient in DNA repair, homologous recombination and (i protein-mediated site-specific recombination. We have also tested the effect on DNA repair of the hbs4755 mutation in combination with mutations in different functions of homologous DNA recombination (recA, recF, recG, recti and addAB). The hbs4755 mutation did not modify the sensitivity of recH and addAB cells to the DNA-damaging agents methylmethane sulphonate (MMS) or 4-nitroquinoline-1-oxide (4NQO), and it only marginally affected recF and recG cells. The hbs4755 mutation blocked intermolecular recombination in recH cells and markedly reduced it (20- to 50-fold) in recF and recG cells, but had no effect on addAB cells. Taken together, these data indicate that the Hbsu protein is required for DNA repair and for homologous DNA recombination.
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  • 70
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    International journal of immunogenetics 8 (1981), S. 0 
    ISSN: 1744-313X
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    Notes: Serum IgG (7S) levels differed significantly for chickens from 10 different inbred lines. Within lines differences between B blood groups were statistically significant.The genetic control of serum IgG was further examined using birds from B complex haplotypes marked at the B locus and the Ir-GAT locus. Birds from each of five subgroup haplotypes (B1B1Ir-GAT-Lo and -Hi, B19B19Ir-GAT-Lo and -Hi, and B2B2Ir-GAT intermediate) were tested for levels of serum IgG at 3, 6, 9, and 21 weeks of age. The rate and level of IgG reached in the serum was more than two-fold greater in the GAT-Hi birds than in the GAT-Lo. The Ir region of the B complex exerts some control over the ontogenesis of IgG, though most of the genetic variation seems not to be B complex associated.
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    Notes: Dorso-ventral vaginal septa were observed in congenic mouse strains. Strain C57BL/10Sn, H-2b, had the highest frequency, 22%; B10.A/SgSn, H-2a, had the lowest frequency, 4%, P 〈0.001. The frequency was about 17% in the F1, females. Septa were found in 16% and 15% of the B10.D2/nSn and B10.BR/SgSn strains, respectively, indicating that at least two loci within the H-2 complex influence its frequency.
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  • 72
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    Notes: The phylogenetic distribution of antigens present on human lymphocytes was investigated by incubating human or simian cells with murine anti-human monoclonal antibodies and then determining the level of reactivity with a radiolabelled anti-murine IgG reagent. The monoclonal antibodies used were specific for a T-cell antigen, lymphoid and lymphoid:myeloid antigens, Ia antigens, and β2 microglobulin. The cells examined included B- and T-lymphoblastoid cell lines and fresh peripheral blood lymphocytes separated by sheep erythrocyte rosetting into T-cell and non T-cell fractions. Results of these studies showed that the antibodies gave complete cross-reactivity with gorilla and chimpanzee cells while B-cell lines of orangutan origin had lost lymphoid and β2 microglobulin markers. Gibbon cells and cells of Old World and New World monkeys reacted strongly only with monoclonal antibodies against Ia antigenic determinants. These Ia antigens were found on the non T-cell fraction of fresh peripheral lymphocytes, on B-cell lines and on some virus induced T-cell tumour lines. Immunoprecipitation analysis using the anti-Ia antibodies showed a degree of molecular diversity on owl monkey and marmoset cells compared to the Ia antigens associated with human cells.
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  • 73
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    Notes: C3H/HeN x NZB/BIN F1 mice were primed and challenged with DNP-Ficoll, a thymic independent (TI-2) antigen. They developed strong IgM and IgG carrier-specific memory responses: DNP-pneumococcal polysaccharide or DNP-haemocyanin challenge did not elicit memory responses. The IgG response component appeared to be inherited dominantly from the C3H/HeN parent. The IgM component resulted from genetic enhancement.
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  • 74
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    Notes: The effect of incompatibility for Mls determinants was studied in lethal graft-versus-host reaction (GVHR) in the mouse. GVHR was induced in adult recipients of the following H-2k strains: (AKR x B10.BR)F1 (MlsB/Mlsb); (C3H x B10.BR)F1 (Mlsc/Mlsb); (CBA/J x B10.BR)F1 (Mlsd/Mlsb) and (CBA/H x B10.BR)F1 (Mlsb). Recipient mice were heavily irradiated and grafted with bone marrow and spleen cells from H-2 compatible B10.BR (H-2k, Mlsb) or H-2 incompatible B10.D2 and B10 donors were normal, while those from B10.BR donors were either normal or pre-immunized against the recipient strains. In all experiments the survival of recipients with Mlsa/Mlsb and Mlsd/Mlsb phenotypes, and only in one experiment of those with Mlsc/Mlsb phenotype was greater and/or the survival time longer than that of recipients expressing only Mlsb. However, late deaths (〉 120 days post grafting) observed after grafting of normal B10.BR cells were more frequent in Mlsd/Mlsb than in Mlsb strains. On the other hand, when B10.BR donor cells were pre-immunized against H-2k compatible (AKR x B10.BR)F1 (Mlsa/Mlsb) or (CBA/J x B10.BR)F1 (Mlsd/Mlsb) strains, the survival time of H-2 incompatible (B10 x B10.BR)F1 (H-2b/k, Mlsb) recipients was longer than when donor cells were pre-immunized against (CBA/H x B10.BR)F1 (Mlsb) strain. We conclude that donor incompatibility for Mlsa or Mlsd or donor-pre-immunization against Mlsa or Mlsd exerts a protective effect on lethal GVHR developed across non-H-2 or H-2 barriers; the protective effect of Mlsc is less efficient or absent. The Mls-induced protective effect shows the following properties: (a) efficiency in vivo correlates with the capacity of the corresponding alleles to stimulate an in vitro MLR; (b) is efficient in either primary or secondary response to other minor antigens; (c) is not H-2 restricted; (d) is nonspecific; (e) disappears late after grafting; (f) with respect to the genetic background, the early protective effect is followed, late after grafting, by an opposite effect which increases the mortality, suggesting that M/s locus determinants are capable of activating several cell populations with different biological functions.
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  • 75
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    Notes: A long-term in vitro grown T cell line derived from a cotton-topped marmoset (Saguinus oedipus)) infected with Herpesvirus saimiri was found to share surface antigens with human amplifier T cells and to augment the capacity of human B cells to secrete immunoglobulin. This is the first demonstration of T/B collaboration across such a large phylogenetic barrier and might have interesting implications for understanding the nature of molecular interactions mediating cell/cell cooperation.
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  • 76
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    Notes: The segregation of genes controlling ECIA susceptibility and the level of immune response to native calf type II collagen were determined in the F2 progeny of matings between WF (RTIu/u; ECIA-susceptible; high responders) and LEW.B3 (RT1n/n; ECIA-resistant; low to intermediate responders). RT1n/n F2 progeny showed resistance to ECIA, low skin test reactivity to type II collagen and intermediate levels of IgG anti-collagen antibodies (-log2 of 6.2 ± 2.6; mean ± SD, n= 10). RT1u/u and RT1u/u F2 progeny were susceptible to ECIA and were high responders to type II collagen by skin testing and IgG antibody titres (-log2 of 12.1 ± 1.3, mean ± SD, n= 26). Although all rats that developed arthritis were also high responders to type II collagen one group of immature F2 progeny, RT1u/u and RT1u/n, showed high anti-collagen immune responses in the absence of detectable arthritis. The data indicate that genes linked to RT1.A control susceptibility to ECIA and at least part of the immune response to native calf type II collagen in WF and LEW.B3 rats.
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  • 77
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    Notes: Mice bearing the I-A subregion mutation bm12 were immunized and challenged with the α-subunit of human adult haemoglobin. Under conditions in which parental B6/Kh mice respond, B6.C-H-2bm12 mice are inhibited nearly 100% in their ability to respond to challenge to the α-chain of haemoglobin. D2.GD mice which express a variant Ae (Eβ) polypeptide of the I-E subregion can respond as well as B10.D2 mice to both subunits (α- and β-) of haemoglobin. These observations as well as other genetic mapping data confirm the I-A mapping of α-chain-specific Ir genes and extend the genetic fine mapping to the Aβ gene within the I-A subregion or a combinatorial Ia determinant generated by an interaction of Aα and Aβ. In addition they implicate the Ia.8 specificity in determining immune responsiveness to α-chain determinants.
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  • 78
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    Notes: Electrophoretic and serologic investigations of C4 in serum samples from a family material, showed that a weak, or partial, inhibition of anti-Rg sera, was an inherited property of some of the C4 haplotype products. The weak Rg activity was strongly, but not absolutely associated with the C4 FI and C4 I haplotypes.
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  • 79
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    Notes: Mouse oocytes at the dictyate stage were tested for the presence of H-2 antigens and Ia antigens, by means of indirect immunofluorescence. Oocytes reacted positively with NIH alloantisera, but were negative with anti-Ia region alloantisera. Results with monoclonal antibodies indicate very low antigen density on oocytes.
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  • 80
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    Notes: A Friend virus-induced tumour of BALB/c (H-2d) origin, HFL/d, was examined for the expression of alien H-2 antigens. The alloantigens on HFL/d were typed by generating CTL in primary MLC with HFL/d as stimulator and measuring reactivity to targets with known H-2 antigens, and confirmed by assessing recognition of HFL/d targets by CTL generated in primary MLC with stimulators expressing known H-2 antigens. Potential cross-reactivities between alloantigens were analysed by cold-target inhibition experiments. BALB/c cells stimulated with HFL/d lysed H-2b targets, and BALB/c anti-H-2b CTL lysed HFL/d; analysis with recombinant haplotypes demonstrated both H-2Kb and H-2Db alien antigens on HFL/d. C57BL/6 (H-2b) cells stimulated with HFL/d recognized H-2Kd, H-2Dd, and an additional determinant unique to HFL/d. (BALB/c x B6)F1 cells also recognised a unique HFL/d determinant not of H-2b or H-2d origin. These unique determinants, which induced a strong cytotoxic response in primary MLC, were not shared by BALB/c or B6 tumours induced by cross-reactive FMR viruses. Thus, HFL/d expressed the K and D antigens of its strain of origin, two typed alien H-2 antigens, and at least one other untyped antigen which may represent an additional H-2 determinant. These studies further demonstrate the utility of examining the reactivity of CTL generated in primary MLC to probe for the presence of alien H-2 antigens.
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  • 81
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    Notes: Properdin factor B is a genetically-controlled polymorphism that can be demonstrated by agarose gel electrophoresis. The Bf gene frequencies were determined for 194 blacks from the south-eastern United States. The frequencies for Bf-F, Bf-S, Bf-F1, and Bf-Sl are 0.626, 0.327, 0.034, and 0.013, respectively. The south-eastern blacks have similar Bf frequencies to the South African blacks; however, both these groups are different from blacks from Boston. The frequency differences between south-eastern blacks and Boston blacks may be due to racial admixture differences found in the two populations.
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    Notes: Fifty-seven members from ten families in which one parent and at least one child have asthma were studies with dilutional skin tests and RAST to grass pollens after determination of HLA haplotypes. We found no direct evidence for linkage of a hypothetical asthma locus with HLA or for a significant association of asthma with HLA haplotypes. Linkage between the HLA loci and a gene or genes which allow for the expression of clinical asthma could neither be proven nor disproven due to the small sample size. All of the asthmatic children had positive dilutional skin tests and RAST, suggesting that atopic asthma may be genetically controlled by the HLA chromosome (chromosome 6). Nonetheless, determination of the histocompatibility antigens can increase the value of predictive risk analysis for asthma. Such a determination may be important in the early identification of a child born to a family with atopic asthma.
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  • 83
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    Notes: T. OHTA; Evolution and Variation of Multigene Families.
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    Notes: The distribution of Glm(f, z, a and x), G3m(b0, b1, b3, b5, c3, c5, g, s and t), A2m (1 and 2) and Km(1) allotypic determinants in a series of Spanish blood donors. The following results were obtained: Gmz,a;gAm1= 0.199; Gmz,a;gAm2= 0.019; Gmz,a;bAm1= 0.081; Gmz,a,x;gAm2= 0.005; Gmf;bAm1= 0.677; Gmz,a;bAm1= 0.019 and Km1= 0.096. The results are similar to those reported for adjacent Basque and Portuguese populations. The origin of the Gmz,a;bAm1 haplotype is unknown.
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    Notes: Serolgoical and immunochemical assays detected antibodies to human la-like antigens in five out of five rabbit antisera to cultured human B lymphoid cells but did not detect them in five bleedings from two rabbits immunized with human peripheral blood lymphocytes. These results, which may reflect differential immunogenicity of la-like antigens from different cells, indicate that the presence of anti la-like antibodies in heterologous antihuman lymphocyte sera is not a general phenomenon as reported recently for animal model systems.
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  • 87
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    Notes: The murine Ly-4 locus is further described with the definition of both Ly-4.1 and 4.2 specificities and the demonstration of linkage of H-3 on Chromosome 2. As has been found for several other Ly-specificities, both Ly-4 specificities are absent from the thymus, but are found on peripheral T and B cells, with B cells having the greatest antigen expression. The expression of both Ly-4 specificities was influenced by an H-2-linked gene, mapping in or near the H-2D locus.
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    Notes: The factorial correspondence analysis, which is a descriptive statistical method, was used to study the preferential associations between locus HLA-A, B, C and Bf alleles. This method does not limit the number ofloci that can be studied and must therefore be considered as an interesting method of studying haplotypic associations within the HLA system.
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  • 89
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    Notes: Erythrocytes that exhibit the rare blood group p phenotype lack the P antigen (globotetraosylceramide) and the Pk antigen (globotriaosylceramide). This phenotype is inherited as an autosomal recessive condition and the red cells of heterozygous individuals, parents and children of p persons, are serologically normal but no chemical analyses of their red cells have been reported. We have studied an unusual family in which all five children exhibit the p phenotype. In addition to the abnormalities described previously, the erythrocytes of four siblings had twice the normal concentration of lactotriaoslyceramide and lactoneotetraosylceramide. These cells also contained 3-5 times as much sialosyllactoneotetraosylceramide and up to a two-fold increase in GM3 ganglioside. The glycolipids of the parents' erythrocytes were normal. Electrophoretic analysis of the glycoproteins of the proposita's erythrocytes revealed no abnormalities, but her erythrocyte membranes contained approximately 35% less galactosamine than normal red cells. This abnormality resulted from a marked decrease in galactosamine that was soluble in chloroformmethanol. The lipid-extracted residue, which contained the glycoproteins, had a normal galactosamine content.
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  • 90
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    Notes: The identity and complete purification of mouse Thymocyte Interaction Modulation Factor (T IMF) is described. Use of silver-stained PAGE methods shows that previous methods of purification yield preparations containing two protein or glycoprotein bands. T IMF activity from H-2k mice can be bound to 15.5.5 monoclonal antibody columns (anti H-2 Dk) but not to 11.4.1 columns (anti H-2 Kk). The activity can be recovered from 15.5.5 columns and runs on PAGE as a single band at approximately 34,000 Daltons. This evidence together with previous evidence relating the activity to H-2 D locus argues that T IMF is a soluble H-2 D antigen fragment equivalent to a papainized H-2 fragment. Additional evidence is presented on an improved method of assay of T IMF activity, on its inactivation by enzymes and serine-esterase inhibitors and of its effect on syngeneic leucocytes and macrophages. It is also shown that T IMF is not appreciably toxic for cells.
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    Notes: One hundred and sixty-five individuals with ragweed rhinitis and asthma were studied. The associations between the HLA system and: Ra3 skin-test response, Ra3 RAST, Ra3/antigen E skin-test reactivity ratio, Ra5 skin-test response, Ra5 RAST, Ra5/antigen E skin-test reactivity ratio, serum IgE levels, antigen E skin-test response, and antigen E RAST were studied. In addition, the influence of the serum IgE levels of these immune parameters were evaluated. No highly significant associations were noted.
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  • 92
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    Notes: We have previously shown that the antibody response to mammalian myoglobins is under genetic control. In the present study we examined antibodies to sperm-whale, Atlantic bottlenosed dolphin, Black Sea dolphin, horse and badger myoglobins, raised in high responder strains of mice, to ascertain whether there is genetic control of antibody affinity to mammalian myoglobins. Using antisera of varying dilutions, the binding to 125I-labelled homologous myoglobins was studied by inhibition with homologous myoglobin over a wide range of inhibitor concentration in a modified Farr assay. The results indicated that there are no large differences between high responder strains of mice in the affinity of antibodies to mammalian myoglobins.
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  • 93
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    Notes: No. Gm, Am, Pi, and Km gene frequency difference has been observed between eighty-seven Caucasin IgA nephropathies and ninety-six controls. But a Km1 significant increase is found in patients with chronic renal failure compared with the total patients group. This data fits for a genetic predisposition factor in IgA nephropathy.
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  • 94
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    Notes: The genetic control of the in vivo growth of the Moloney virus-induced BALB/c lymphoma YC8 was studied in F1 hybrids between BALB/c and several strains differing at the MHC and/or at the level of non-H-2 genes. Parental strains of the B10 and C3Hf but not of A, BALB/c or DBA/2 backgrounds introduced a significant resistance to the growth of 102 YC8 cells (a dose able to kill 100% of BALB/c mice) in semisyngeneic hosts. This resistance appeared to be due to non-H-2 genes although a modulation of the tumour growth by genes encoded by the MHC was also evident. The study of backcrosses between susceptible BALB/c and resistant (BALB/c x B10.BR)F1 crosses revealed that 83% of animals developed lethal tumors after injection of 102 YC8 cells. This high frequency of tumour takes was not linked to genes of the MHC. Adult thymectomy plus sublethal irradiation was able to abrogate the resistance of (BALB/c x B10.BR) or (BALB/c x B10.RIII)F1 mice to YC8 growth. Since the injection of silica also impaired the resistance to YC8, we tentatively concluded that the genetic control of resistance to YC8 is mediated both by T cells and macrophage-like cells.
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    Notes: Mice of independent haplotypes and several recombinant inbred strains were immunized with highly purified preparations of either the α-chain or β-chain subunit of human adult haemoglobin. Cells from the sensitized lymph nodes were challenged in vitro with the appropriate subunit (or in some cases both chains) and cell proliferation assessed by 3H-thymidine incorporation. Mice of the H-2b and H-2d haplotypes were high responders to α-chain while mice of the H-2f, H-2j, H-2k, H-2r, H-2s, H-2u, and H-2v haplotypes were low responders. The low responsiveness of B10.A(4R) and B10.MBR and the high responsiveness of B10 indicated that the Ir gene(s) determining responsiveness to the α-chain subunit resides in the I-A subregion of the mouse major histocompatibility complex. Mice of the H-2d, H-2f, and H-2s haplotypes were high respoders and H-2b, H-2f, H-2q, and H-2u haplotype mice were low responders to β-chain. H-2k, H-2p, H-2r, and H-2v haplotype mice were intermediated responders to β-chain. The low responsiveness of B10.S(8R) and B10.TL and the high responsivenes of B10.S(9R) mapped the Ir gene(s) to β-chain to the I-A subregion. Data collected from challenging high responder cells with both subunits indicated that α-chain and β-chain do not crossreact. These results are discussed in reference to earlier observations suggesting that the low responsiveness of some strains of mice to priming and challenging using the intact haemoglobin molecule might be due to a negative regulatory influence mediated by one of the subunits. In the absence of this influence these mice would respond normally.
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    Notes: An attempt to determine association between immunoglobulin allotypes and Schistosoma japonica infection was made. The results show that no apparent relationship exists.
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    Notes: Diet and Resistance to Disease; Advances in Experimental Medicine and Biology. Vol. 135. M. Phillips and A. Baetz
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    Notes: The antigenic specificities of six (1-6) IgG allotypes of the domestic mink were tested in the sera of closely related species of Mustelidae family and distant mammalian species. It was found that allotypes 1 and 5 are ancient. Their antigenic specificities were established not only in Mustelidae, but also in other taxonomic orders of mammals. Allotypes 3 and 2 are phylogenetically younger; they were detected only in Mustelidae. Allotypes 4 and 6 appear to be unique to the domestic mink.The instantaneous evolutionary emergence of complex allotypes 4 and 6 is difficult to explain by a rapid accumulation of gene point mutations during phylogenesis. Activation in the domestic mink of those immunoglobulin genes, which are silent or poorly expressed in closely related Mustelidae, is suggested as a more plausible explanation.
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    Oxford, UK : Blackwell Publishing Ltd
    International journal of immunogenetics 8 (1981), S. 0 
    ISSN: 1744-313X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: In the V/V type of bovine erythrocytes (BE), the antigen IM and the Thomsen-Friedenreich (TF) receptors, which reacted with a TF-specific agglutinin from human sera and the lectin from Helix pomatia, were all destroyed by treatment with trypsin. Unlike the V/V, in the F/F type of BE the antigen IM and these TF receptors were not only not destroyed by trypsin, but this enzyme treatment was, in fact, necessary to make them amenable to the corresponding agglutinins.Another TF receptor, which reacted with the lectin from peanuts, was identified on a trypsin-resistant component of all BE and independently of the FV genotypes.
    Type of Medium: Electronic Resource
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