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  • 1
    Electronic Resource
    Electronic Resource
    Sequential tunneling due to intersubband scattering in double-barrier resonant tunneling devices (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 212-214 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetoquantum oscillations in the tunnel current of double-barrier n-GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi-bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99522
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  • 2
    Electronic Resource
    Electronic Resource
    Edge effects in a gated submicron resonant tunneling diode (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 2508-2510 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have investigated the current-voltage [I(V)] characteristics of a gated GaAs/(AlGa)As resonant tunneling diode. As the negative gate voltage is progressively increased I(V) becomes asymmetric. In particular the peak-to-valley ratio in forward bias is decreased from (approximately-equal-to)20 to (approximately-equal-to)1, but in reverse bias remains constant (approximately-equal-to)20. This arises from a lateral variation of the voltage drop across the emitter tunnel barrier, which in forward bias leads to a smearing of the resonance. We discuss the relationship between our experiment and the low peak-to-valley ratios of two-terminal submicron resonant tunneling diodes observed by other groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.106949
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  • 3
    Electronic Resource
    Electronic Resource
    Submicrometer resonant tunnelling diodes fabricated by photolithography and selective wet etching (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 1124-1126 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new process based on photolithography and selective wet etching has been used to fabricate small area resonant tunnelling diodes. The low-temperature current-voltage (I-V) characteristics of diodes with conducting widths less than 0.1 μm show additional peaks due to 1D lateral quantum confinement. We observe a pronounced asymmetry in I-V which we explain in terms of tunnelling from emitter states which have a different degree of lateral quantum confinement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.112117
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  • 4
    Electronic Resource
    Electronic Resource
    Identification of the ligand-binding domain of the surface-located fibrinogen receptor (clumping factor) of Staphylococcus aureus (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Molecular microbiology 16 (1995), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The ability of Staphylococcus aureus to bind to fibrinogen and fibrin is believed to be an important factor in the initiation of foreign-body and wound Infections. Recently, we reported the cloning and sequencing of the gene clfA encoding the fibrinogen receptor (clumping factor, ClfA) of S. aureus strain Newman and showed that the gene product was responsible for the clumping of bacteria in soluble fibrinogen and for the adherence of bacteria to solid-phase fibrinogen. This was confirmed here by showing that antibodies raised against purified Region A inhibited both of these properties. Also, immunofluorescent microscopic analysis of wild-type Newman and a clfA::Tn917 mutant of Newman with anti-ClfA Region A sera confirmed that Region A is exposed on the bacterial cell surface. Furthermore, polystyrene beads coated with the Region A protein formed clumps in soluble fibrinogen showing that the ClfA protein alone is sufficient for the clumping phenotype. Western immunoblotting with anti-ClfA Region A antibodies identified the native ClfA receptor as a 185 kDa protein that was released from the cell wall of S. aureus by lysostaphin treatment. A single extensive ligand-binding site was located within Region A of the ClfA protein. Truncated ClfA proteins were expressed in Escherichia coli. Lysates of E. coli and proteins that had been purified by affinity chromatography were tested for (i) their ability to bind fibrinogen in Western ligand blotting experiments, (ii) for their ability to inhibit clumping of bacteria in fibrinogen solution and adherence of bacteria to solid-phase fibrinogen, and (iii) for their ability to neutralize the blocking activity of anti-ClfA Region A antibody. These tests allowed the ligand-binding domain to be localized to a 218-residue segment (residues 332-550) within Region A.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2958.1995.tb02316.x
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular characterization of the clumping factor (fibrinogen receptor) of Staphylococcus aureus (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Molecular microbiology 11 (1994), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Four mutants of Staphylococcus aureus strain Newman that were defective in the fibrinogen receptor (clumping factor) were isolated by transposon Tn917 mutagenesis. Southern hybridization analysis of the mutants identified transposon-host DNA junction fragments, one of which was cloned and used to generate a probe to identify and clone the wild-type clumping factor locus (clfA). The mutants failed to form clumps in soluble fibrinogen and adhered poorly to polymethylmethacrylate (PMMA) coverslips coated with fibrinogen. A single copy of the clfA gene, when introduced into the chromosome of the mutant strains, fuily compiemented the ciumping deficiency of these strains and restored the ability of these mutants to adhere to fibrinogen-coated PMMA. in addition, the cloned clfA gene on a shuttle plasmid aiiowed the weakiy ciumping strain 8325-4 to form clumps with the same avidity as the wild-type strain Newman and also significantly enhanced the adherence of 8325-4 strains. Thus the formation of clumps in soluble fibrinogen correlated with adherence of bacteria to solid-phase fibrinogen. The clfA gene encodes a fibrinogen-binding protein with an apparent molecular mass of c. 130 kDa. The amino acid sequence of the protein was deduced from the DNA sequence; it was predicted that a 896 residue protein (molecular mass 92 kDa) would be expressed. The putative ClfA protein has features that suggest that it is associated with the ceil surface. Furthermore it contains a novel 308 residue region comprising dipeptide repeats predominantly of Asp and Ser ending 28 residues upstream from the LPXTG motif common to wall-associated proteins. Significant homology was found between the ClfA protein and the fibronectin-binding proteins of S. S. aureus, particularly in the N-and C-termini.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2958.1994.tb00304.x
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  • 6
    Electronic Resource
    Electronic Resource
    The coagulase of Staphylococcus aureus 8325-4. Sequence analysis and virulence of site-specific coagulase-deficient mutants (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Molecular microbiology 4 (1990), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The sequence of the coagulase gene (coa) from Staphylococcus aureus strain 8325-4 is reported. The deduced amino acid sequence of the coagulase protein is compared with previously reported sequences of coagulases from strains 213 and BB. The secreted mature forms of coagulase proteins are composed of three distinct segments: (i) the N-terminal 150–270 residues, which are c. 50% identical, (ii) a central region with high (〉90%) residue identities, and (iii) a C-terminal region composed of repeated 27-amino-acid residue sequences. The variable N-terminal sequences are probably responsible for antigenic differences among coagulases of different serotype. The region of coagulase which binds to prothrombin and activates it to form staphylothrombin is also located in the N-terminal half of the protein.A site-specific substitution mutation in the coa gene, which abolished plasma clotting activity, was isolated by recombinational allele-replacement in strains 8325-4 and M60. The Coa− mutants did not show diminished virulence in subcutaneous and intramammary infections of mice. No evidence for a role for coagulase in virulence of toxigenic or non-toxigenic strains was obtained. This contradicts findings of several groups using Coa− mutants generated by chemical mutagenesis and suggests that the earlier results were obtained with strains that had suffered additional mutations in virulence-related genes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2958.1990.tb00606.x
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  • 7
    Electronic Resource
    Electronic Resource
    Cryptic α-toxin gene in toxic shock syndrome and septicaemia strains of Staphylococcus aureus (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Molecular microbiology 4 (1990), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: The majority of clinical isolates of Staphylococcus aureus that produce toxic shock syndrome toxin-1 (TSST-1) fail to express α-toxin, despite having a copy of the hla gene in the chromosome. The hla gene was cloned from an Hla- TSST-1+ strain, Todd 555, which had been isolated from a case of toxic shock syndrome in the USA. Of the 630 bases of the Todd 555 gene sequenced, 46 differed from the hla gene sequence of strain Wood 46. The defect in α-toxin expression was shown to be due to a nonsense mutation which converted a CAG glutamine codon in the equivalent position in the functional Wood 46 sequence to a TAG stop codon. The same mutation was present in the hla gene cloned from a human septicaemia strain (V37) isolated in Dublin. The nonsense mutation of Todd 555 was suppressed by the supE 44 mutation in Escherichia coli resulting in haemolytic activity in cell lysates. Hybrid hla genes were formed by splicing fragments of hla from Todd 555 and Wood 46. Expression of one such chimaeric hla gene in S. aureus demonstrated that the Todd 555 hla gene has a functional agr-regulated promoter. The silent hla gene may be a cryptic gene in S. aureus.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2958.1990.tb02044.x
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  • 8
    Electronic Resource
    Electronic Resource
    Analysis of the reduction in expression of tetracycline resistance determined by transposon Tn10 in the multicopy state (1981)
    Springer
    Molecular genetics and genomics 182 (1981), S. 171-177 
    Details
    ISSN: 1617-4623
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The tet genes of transposon Tn10 have been mapped in a 2,200 bp DNA sequence by analysing deletion and Tn5 insertion mutations. When the tet genes were present on multi-copy plasmids the level of resistance expressed was about ten-fold lower than that determined by a single copy of Tn10 in the E. coli chromosome. The 36K tet protein known to be encoded by R100 in E. coli minicells was not detected when they harboured a multicopy tet plasmid. However, normal high levels of resistance were expressed when the tet genes were recombined into the host chromosome as part of a lambda lysogen, showing that the multicopy effect was phenotypic. Most of the Tn5 insertions and deletions in tet which caused Tcs mutations also prevented expression of high level Tcr from a chromosomal Tn10 element present in the same cell. Only those insertions in the promoter-proximal 90–130 bp of a 1,275 bp HindII fragment known to carry the gene encoding the 36K tet protein did not reduce the single copy Tn10 resistance level. A gene fusion system that results in the constitutive synthesis of β-galactosidase from a tet promoter has been used to assay tet repressor activity. The basal (uninduced) β-galactosidase level in cells carrying multicopy tet plasmids was 10–20 fold lower than those carrying a single copy. The tet:: Tn5 mutants defective in the trans-dominant multicopy effect still made normal amounts of tet repressor showing that repressor overproduction was not responsible for this effect. In addition a repressor-defective constitutive mutant did not exhibit a higher resistance level when located on a multicopy plasmid vector. We postulate that a regulatory mechanism recognises the amino-terminus of the tet structural gene product when attempts are being made to overproduce the protein and prevents further translation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00422786
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  • 9
    Electronic Resource
    Electronic Resource
    Tetracycline-sensitive mutants of the F-like R factors R 100 and R 100-1 (1975)
    Springer
    Molecular genetics and genomics 137 (1975), S. 85-88 
    Details
    ISSN: 1617-4623
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The majority of tetracycline-sensitive (Tets) mutants of R100 and R100-1 are multisite (deletion) mutants. About 50% of these are also transfer-deficient, indicating that the Tetr locus is closely linked to the transfer genes. Tets mutants with single-site lesions are also described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00332542
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  • 10
    Electronic Resource
    Electronic Resource
    R factor-mediated tetracycline resistance in Escherichia coli K12 dominance of some tetracycline sensitive mutants and relief of dominance by deletion (1976)
    Springer
    Molecular genetics and genomics 143 (1976), S. 339-344 
    Details
    ISSN: 1617-4623
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Strains of Escherichia coli K12 heterozygous for the R100-1 tetracycline resistance region were constructed. They carried the wild-type Tetr genes in the chromosome and single site Tets mutations on plasmids. Some heterozygotes could not express tetracycline resistance fully after induction. The mutant tet allele was thus partially dominant. When heterozygotes carrying the dominant tet mutant were plated on agar containing 50 μg/ml tetracycline, mutants which grew normally occurred at a frequency of 1–4×10-4. Analysis of these dominance relief mutants showed that in 53/56 isolates the dominant tet allele was lost forming either Tra+ or Tra- deletion mutants of the plasmid. The mutation frequency was not affected either by the host chromosomal recA mutation or by the temperature of growth of the culture.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00269413
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