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  • 1
    ISSN: 1432-1017
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1017
    Keywords: Viroids ; Thermodynamics ; Kinetics ; Hydrodynamics ; Function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Viroids are an independent class of plant pathogens which are distinguished from viruses by the absence of a protein coat and by their unusually small size. They are single-stranded circular RNAs composed of about 360 nucleotide residues. Sequence analysis and physicochemical studies of the potato spindle tuber viroid (PSTV) have shown that, as a result of intra-molecular base pairing, viroids form a unique rod-like secondary structure which is characterized by a serial arrangement of double-helical sections and internal loops. There is no indication for an additional tertiary structure because all parts of the molecule are freely accessible to ligand interaction. During the denaturation all of the native base pairs of viroids are dissociated in one highly cooperative transition, and in the same process very stable hairpins which are not present in the native structure are newly formed. Most of the properties of the structure and structural transitions of PSTV have been found also in citrus exocortis viroid, chrysanthemum stunt viroid and four different viroid-like RNAs associated with the cadang-cadang disease. The close similarity between these viroids is more expressed in the overall structure and in thermodynamic and functional domains than in the primary sequence. The stiffness of all viroids can be described by an unique persistence length of 300 å. Characteristically, regions of premelting, regions of stable hairpins, and the sequence UACUACCCGGUGG which is opposite to one of the stable hairpins, are the most conservative sequences in the molecules. Current hypotheses about the function of viroids are discussed on the basis of their structural and thermodynamic features. The suggestion that viroid RNA has features similar to DNA has been supported by the finding that they are replicated in vitro by the DNA-dependent RNA polymerase II of the host plant. The highly conserved sequence in viroids mentioned above corresponds very closely to a segment at the 5′-end of the small nuclear RNA U1 of eukaryotes. Because this segment is discussed in recent models, to be involved in the splicing process, a hypothesis is proposed in which viroids interfere with the splicing process leading to a pathogenic misregulation of mRNA processing.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical and applied genetics 28 (1958), S. 256-256 
    ISSN: 1432-2242
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 164 (1961), S. 341-358 
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Zusammenfassung Bestrahlt manF-Zentren in Alkalihalogeniden bei Zimmertemperatur mit Licht von der Wellenlänge ihrer Absorptionsbande, so wandeln sie sich in eine Reihe von Folgezentren um; das erste dieser Folgezentren ist dasM-Zentrum. In der vorliegenden Arbeit wird gezeigt, daß dieses Störzentrum kein meßbares Elektronen-spinresonanz-(ESR-) Signal liefert. Hierzu wurden drei voneinander unabhängige Meßmethoden angewendet: 1. Die aus ESR-Absorptionsmessungen bestimmbare Spindichte wurde mit der optisch gemessenenF-Zentren-Konzentration vor und nach derM-Umwandlung verglichen. 2. Kristalle mit ausgerichtetenM-Zentren wurden auf eine Anisotropie ihrer ESR untersucht. 3. ESR-Dispersionssignale wurden bei verschiedenen Mikrowellenleistungen gemessen und analysiert. Keine dieser Messungen lieferte einen Anhaltspunkt für die Existenz einer ESR desM-Zentrums, so daß dasM-Zentrum mit großer Wahrscheinlichkeit als diamagnetisch zu betrachten ist. Dieses Resultat spricht gegen dasM-Zentren-Modell vonSeitz undKnox; es läßt sich aber gut mit dem vonvan Doorn-Haven undPick vorgeschlagenen Modell vereinbaren.
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  • 5
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Zusammenfassung Es wird eine Arbeitsweise zur Herstellung lichtelektrischer Emissionsbilder angegeben, welche auch bei tieferen Temperaturen die Kristallstruktur von Nickeloberflächen durch Bariumbedampfung sichtbar macht. Zusätzlich zu der bekannten glühelektrischen Methode der elektronenoptischen Abbildung besteht durch diese Methode heute die Möglichkeit, von Zimmertemperatur bis zur Schmelztemperatur der höchst schmelzenden Metalle elektronenoptische Kristallstrukturbilder zu erzeugen.
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  • 6
    ISSN: 1420-9071
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Zusammenfassung β-Pyrrolo-l-alanin, dessen Synthese beschrieben ist, hemmt das Wachstum vonE. coli B auf synthetischen Nährböden. Die Hemmung wird vonl-Phenylalanin undl-Tyrosin vollständig, vonl-Tryptophan teilweise, nicht aber von Histidin aufgehoben. Stämme vonE. coli, die gegenβ-Pyrrolo-l-alanin bzw.p-Fluorophenyl-dl-alanin resistent sind, zeigen gekreuzte Resistenz.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1420-9039
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Physics
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1615-6102
    Keywords: Funaria protonemata ; Polar cell wall formation ; Membrane rosettes ; Freeze fracturing ; Growth inhibition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary In youngFunaria protonemata the influence of various inhibitors and treatments on cell elongation, fine-structure, and particle rosettes within the plasma membrane, putative parts of cellulose synthase complexes, was investigated. Cycloheximide (3×10−5M) inhibited growth, reduced the number of rosettes and evened the gradient of rosette distribution at the beginning of treatment. The cell fine-structure was unaffected. Actinomycin D (10−5M and 10−4) caused an initial but transient decrease in rosette number. Alterations in cell elongation and fine-structure have not been observed. Application of 2.6-dichlorobenzonitrile (10−5 M) for some minutes reduced the number of rosettes remarkably, while cell elongation seemed to be normal after the filaments had been transferred back to normal medium. An incubation of 2 h or longer stopped growth and caused cells to burst. The number of rosettes then rose to about 50% of the control values. When applied for 7 h biofluor (5×10−4 M) promoted growth slightly, but generally it retarded it when used for a longer time. It did not markedly affect the number of rosettes. A short heat stock stopped elongation, caused the disappearance of rosettes and affected the structure of the mitochondria and of the Golgi apparatus. Plasmolysed cells did not grow and, initially, did not have rosettes. At reduced turgor, wider cells are formed. Freeze fracturing under UHV conditions and shadowing at very low specimen temperature revealed a small, central depression in the 8 nm rosette particles, suggesting that they are composed of subunits. Our results provide further evidence that the rosettes are parts of the cellulose synthase complexes. Their existence clearly depends on protein synthesis and on the constitution of the plasma membrane, but not on cellulose crystallization.
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  • 9
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1573-4986
    Keywords: sialic acid analogues ; CMP-glycosides ; sialyltransferases ; resialylation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We present kinetic studies on the enzymatic transfer of several synthetic sialic acid analogues, modified at C-5, to distinct glycoprotein glycans by sialytransferases differing in acceptor- and linkage-specificity. Biochemical properties of sialic acids were modified by introducing formyl-, trifluoroacetyl-, benzyloxycarbonyl-, and aminoacetyl-groups to the amino group at C-5 of neuraminic acid. The latter substitution renders the corresponding α-glyocoside resistant towards sialidases. The respective CMP-sialic acid analogues were prepared by CMP-sialic acid synthase with a yield of 13–55%. The kinetic parameters of several sialyltransferases for the 5-substituted CMP-glycosides differed significantly. Relative to parent CMP-NeuAc, reaction rates of human- and rat liver Galβ1, 4GlcNAc α2,6-sialyl-transferases ranged from 50 to 170%, of GalNAc α2,6-sialyltransferases from 40–140%, and of Galβ1,3Gal-NAc α2,3-sialyltransferase from 20–50%. Resialylation of asialo-α1-acid glycoprotein by 5-N-formyl- and 5-N-aminoacetyl-neuraminic acid employing rat liver Galβ1,4GlcNAc α2,6-sialyltransferase proceeded to about 80% of galactose sites which is identical to the extent achieved with parent NeuAc. According to our data, neosialoglycoconjugates which carry sialic acids modified at theN-acetyl group can be prepared for structure-function analysis, as this position seems crucial for recognition of adhesion proteins and influenza viruses.
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