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A Danchin

Publications and source records attributed to A Danchin.

At least 199 records · Page 11Linked to original sources
PubMed1979

Is a metabolic control for the doubling of the macromolecule synthesizing machinery possible.

The usual models for cell multiplication only involve macromolecular events and ignore the influence of metabolism. The conjecture presented here shows how metabolism could be involved. Separate pathways might cooperate for the synthesis of a small molecule, acting as positive control of several macromolecular syntheses, including stable RNA synthesis. A detailed analysis of the available data showing an influence of metabolites on macromolecular syntheses suggests that two main pathways might cooperate, one involving serine and folic acid metabolism and the second involving threonine and the branched chain aminoacids. Experiments are suggested to challenge the conjecture.

Amino Acids↗
PubMed1979

Cyclic AMP as a modulator of polarity in polycistronic transcriptional units.

The degree of natural polarity in the lactose and galactose operons of Escherichia coli is affected by adenosine 3',5'-cyclic monophosphate (cAMP). This effect, mediated by the cAMP receptor protein, is exerted at sites distinct from the promoter. Experiments performed with a mutant bearing a thermosensitive rho factor activity indicate that cAMP relieves polarity by interfering with transcription termination. Conflicting results in the literature concerning the role of cAMP receptor protein and cAMP in galactose operon expression can be reconciled by the findings that cAMP stimulates the expression of operator distal genes without significantly affecting the proximal genes. Therefore, it appears necessary to reevaluate the classification of the galactose operon as exhibiting cAMP-mediated catabolite repression at the level of transcription initiation.

Cyclic AMP↗
PubMed1978

Correlation between the serine sensitivity and the derepressibility of the ilv genes in Escherichia coli relA- mutants.

Upon addition of excess one carbon metabolites (including serine)bacteria stop growing because of isoleucine starvation. After such treatment stringent bacteria rapidly resume normal growth whereas relaxed mutants remain unable for some time to grow. We show here that this is due to a lack of derepressibility of ilv genes after the starvation period. Results are also presented which show that RNA polymerase structural mutants may be selected among the clones resistant to a mixture of serine, methionine and glycine, in relA- strains. Finally circumstancial evidence suggests that the one carbon metabolism may be involved in a process controlling isoleucine metabolism.

DNA-Directed RNA Polymerases↗
PubMed1978

Structural and exchange properties of "Co(III)-phenanthroline-ATP": a labeling reagent for the active site of ATPases.

This paper reports on 1H and 31P NMR as well as EPR measurements of the labeling reagent of ATPase sites, "Co(III)-(phen)-ATP." This complex is found to be paramagnetic, as deduced both from its EPR spectrum and from the significant broadening, though almost unshifted, proton and phosphorus resonances. This paramagnetism is a result of the incorporation of the superoxide free-radical anion in the coordination sphere of the trivalent cobalt ion. Evidence for the presence of superoxide in the complex is based on competition experiments with cyanide, which is able to displace the superoxide anion. The latter was identified by its inducing effect on the photoreactivity of luminol. The displacement of superoxide by cyanide was accompanied by the abolition of the paramagnetism of the complex. The relative distances between the protons and phosphorus atoms of ATP and the superoxide anion in the complex were calculated using the NMR line-broadening data. Structural models compatible with the experimental results are proposed. Under conditions of excess of adenine nucleotides or phenanthroline, the coordinated ATP molecule becomes exchangeable. This phenomenon is attributed to the labilization of the cobaltic ion ligands induced by the superoxide anion.

Adenosine Triphosphatases↗
PubMed1978

Formylation of initiator tRNA methionine in procaryotic protein synthesis: in vivo polarity in lactose operon expression.

Eucaryotic and procaryotic organisms differ in two aspects of their translation machinery: polycistronic messengers are expressed as a sequence of individual proteins only in procaryotes, and the initiation of protein synthesis proceeds with an initiator tRNA which is found to be modified (formylated) in procaryotes and not in eucaryotes. In the present study, we show that formylation is required in vivo for the coordinate expression of the Escherichia coli lactose operon. Our experiments are consistent with a translation mechanism using dissociated ribosomes at the 5' end of the mRNA in a reaction that is only weakly dependent on formylation at this initiation step; the ribosomes then travel along the messenger and can reinitiate after the intracistronic barrier without dissociation. This latter initiation step is strongly dependent on the level of formylation: a low level of the formyl group, obtained by the antifolic agent trimethoprim, induces a strong polarity in the expression of the lactose operon. There exist mutant strains in which this polarity is much less apparent than in the wild type. We show here that such is the case of rpsL mutants. Ribosomes mutated in the S12 protein (rpsL) are found to be much more easily dissociated than the wild type. This might explain why the expression of the lactose operon on rpsL strains remains coordinated when the intracellular level of formylation is decreased.

Acetyltransferases↗
PubMed1977

A new technique for selection of sensitive and auxotrophic mutants of E. coli: isolation of a strain sensitive to an excess of one-carbon metabolites.

An E. coli strain deleted in the region malasd is used for the selection of conditional or auxotrophic mutants. Thermosensitive and auxotrophic strains have thus been isolated on plates. After selection in liquid medium, a strain has been isolated which is sensitive to excess one-carbon metabolites. It carries two mutations, smg A1 (near metA and arg H), probably identical to relC, and smgB (between asn and ilv), probably part of the E. coli membrane ATPase.

Escherichia coli↗
PubMed1976

Toward an understanding of the formylation of initiator tRNA methionine in prokaryotic protein synthesis. I. In vitro studies of the 30S and 70S ribosomal-tRNA complex.

Formation of the 30S-tRNA initiation complex of Escherichia coli with nonformylated initiator tRNA is stimulated by all three initiation factors and is messenger dependent, whereas the complex formation involving the 70S ribosomes is strongly inhibited by initiation factors when the nonformylated species is used. When the 30S-Met-tRNAfMet complex is first formed and the 50S ribosomal subunit added subsequently, there is no significant inhibition by initiation factors and the nonformylated initiator tRNA is puromycin reactive. This leads to the conclusion that the formylation of the methionyl initator tRNA is only obligatory when polypeptide synthesis is initiated by nondissociated 70S ribosomes.

Binding Sites↗
PubMed1976

Toward an understanding of the formylation of initiator tRNA methionine in prokaryotic protein synthesis. II. A two-state model for the 70S ribosome.

The 70S ribosomes can select the proper initiator tRNA between Met-tRNAfMet and fMet-tRNAfMet. Experiments on binding and on formation of aminoacylpuromycin, as a function of magnesium, potassium, or initiation factors, suggest a two-state equilibrium for 70S particles, involving a minor, active conformation and a major one which is not readily active. The formyl group would act as a specific trigger to select the active conformation. Experimental results are interpreted following this simple model and equilibrium parameters, together with kinetic constants of the peptidyltransferase activity, are presented.

Binding Sites↗
PubMed1975

[Technical problems in the nuclear magnetic resonance of nucleic acids].

The application of nuclear magnetic resonance to nucleic acids is discussed here, in terms of the proposed study of their structural, conformational and dynamic aspects. Relaxation techniques, using for example paramagnetic ions as localised probes, provide information about dynamic behaviour ; high resolution spectroscopy about three-dimensional structure, using proton or carbon 13 resonance for the overall study, and phosphorus 31 resonance for the study of phosphate polyribose chain.

Carbon Isotopes↗