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G Gouesbet

Publications and source records attributed to G Gouesbet.

17 recordsLinked to original sources
PubMed2000

Modeling maps by using rational functions

Rational functions are not very useful for obtaining global differential models because they involve poles that may eject the trajectory to infinity. In contrast, it is here shown that they allow one to significantly improve the quality of models for maps. In such a case, the presence of poles does not involve any numerical difficulty when the models are iterated. The models then take advantage of the ability of rational functions to capture complicated structures that may be generated by maps. The method is applied to experimental data from copper electrodissolution.

Journal Article↗
PubMed1996

Characterization of the Erwinia chrysanthemi osmoprotectant transporter gene ousA.

Growth of Erwinia chrysanthemi in media of elevated osmolarity can be achieved by the uptake and accumulation of various osmoprotectants. This study deals with the cloning and sequencing of the ousA gene-encoded osmoprotectant uptake system A from E. chrysanthemi 3937. OusA belongs to the superfamily of solute ion cotransporters. This osmotically inducible system allows the uptake of glycine betaine, proline, ectoine, and pipecolic acid and presents strong similarities in nucleotide sequence and protein function with the proline/betaine porter of Escherichia coli encoded by proP. The control of ousA expression is clearly different from that of proP. It is induced by osmotic strength and repressed by osmoprotectants. Its expression in E. coli is controlled by H-NS and is rpoS dependent in the exponential phase but unaffected by the stationary phase.

Amino Acid Sequence↗
PubMed1994

Pipecolic acid is an osmoprotectant for Escherichia coli taken up by the general osmoporters ProU and ProP.

Exogenously supplied L-pipecolic acid was accumulated by Escherichia coli cells and protected them while growing at inhibitory osmolarity. Using specific uptake mutants and competitive assays, we established that the imino acid enters the cells through the ProP and ProU systems with Km values of 225 and 53 microM, respectively. Surprisingly, in spite of the requirement for the wild-type proX gene for osmoprotective ability, no binding activity of labelled pipecolate with the periplasmic protein encoded by proX could be detected. In an attempt to demonstrate whether the two porters (ProP and ProU) are the only carriers involved in osmoregulation, a variety of molecules known for their intracellular osmolarity-dependent accumulation in various organisms were investigated. N-Dimethylproline (proline betaine), N-dimethylglycine, homobetaine (beta-alanine betaine), gamma-butyrobetaine and dimethylsulfoniopropionate were found to be capable of promoting the growth of osmotically stressed E. coli. All of these molecules enter bacterial cells via ProP and ProU porters. None of the osmoprotectants except N-dimethylproline was able to bind the periplasmic protein encoded by proX, while this protein was necessary for their uptake. Apparently, ProP and ProU are the sole osmoporters involved in osmolyte influx into E. coli cells.

Amino Acid Transport Systems↗
PubMed1994

Osmoadaptation in rhizobia: ectoine-induced salt tolerance.

After having shown that ectoine (a tetrahydropyrimidine) displays osmoprotective properties towards Escherichia coli (M. Jebbar, R. Talibart, K. Gloux, T. Bernard, and Blanco, J. Bacteriol. 174:5027-5035, 1992), we have investigated the involvement of this molecule in the osmotic adaptation of Rhizobium meliloti. Ectoine appeared almost as effective as glycine betaine in improving the growth of R. meliloti under adverse osmotic conditions (0.5 M NaCl). Moreover, improvement of growth of rhizobial strains insensitive to glycine betaine was also observed. Ectoine transport proved inducible, periplasmic protein dependent, and, as shown by competition experiments, distinct from the transport of glycine betaine. Medium osmolarity had little effect on the uptake characteristics, since the rate of influx increased from 12 to only 20 nmol min-1 mg of protein-1 when NaCl concentrations were raised from 0 to 0.3 or 0.5 M, with a constant of transport of 80 microM. Natural-abundance 13C-nuclear magnetic resonance and radiolabelling assays showed that ectoine, unlike glycine betaine, is not intracellularly accumulated and, as a consequence, does not repress the synthesis of endogenous compatible solutes (glutamate, N-acetylglutaminylglutamine amide, and trehalose). Furthermore, the strong rise in glutamate content in cells osmotically stressed in the presence of ectoine suggests that, instead of being involved in osmotic balance restoration, ectoine should play a key role in triggering the synthesis of endogenous osmolytes. Hence, we believe that there are at least two distinct classes of osmoprotectants: those such as glycine betaine or glutamate, which act as genuine osmolytes, and those such as ectoine, which act as chemical mediators.

Amino Acids, Diamino↗
PubMed1993

Osmotic repression of anaerobic metabolic systems in Escherichia coli.

The influence of the osmolarity of the growth medium on anaerobic fermentation and nitrate respiratory pathways was analyzed. The levels of several enzymes, including formate dehydrogenase, hydrogenase, and nitrate reductase, plus a nickel uptake system were examined, as was the expression of the corresponding structural and regulatory genes. While some functions appear to be only moderately affected by an increase in osmolarity, others were found to vary considerably. An increase in the osmolarity of the medium inhibits both fermentation and anaerobic respiratory pathways, though in a more dramatic fashion for the former. fnr expression is affected by osmolarity, but the repression of anaerobic gene expression was shown to be independent of FNR regulatory protein, at least for hyd-17 and fdhF. This repression could be mediated by the intracellular concentration of potassium and is reversed by glycine betaine.

Anaerobiosis↗
PubMedDate not supplied

Hypothesis: hyperstructures regulate bacterial structure and the cell cycle.

A myriad different constituents or elements (genes, proteins, lipids, ions, small molecules etc.) participate in numerous physico-chemical processes to create bacteria that can adapt to their environments to survive, grow and, via the cell cycle, reproduce. We explore the possibility that it is too difficult to explain cell cycle progression in terms of these elements and that an intermediate level of explanation is needed. This level is that of hyperstructures. A hyperstructure is large, has usually one particular function, and contains many elements. Non-equilibrium, or even dissipative, hyperstructures that, for example, assemble to transport and metabolize nutrients may comprise membrane domains of transporters plus cytoplasmic metabolons plus the genes that encode the hyperstructure's enzymes. The processes involved in the putative formation of hyperstructures include: metabolite-induced changes to protein affinities that result in metabolon formation, lipid-organizing forces that result in lateral and transverse asymmetries, post-translational modifications, equilibration of water structures that may alter distributions of other molecules, transertion, ion currents, emission of electromagnetic radiation and long range mechanical vibrations. Equilibrium hyperstructures may also exist such as topological arrays of DNA in the form of cholesteric liquid crystals. We present here the beginning of a picture of the bacterial cell in which hyperstructures form to maximize efficiency and in which the properties of hyperstructures drive the cell cycle.

Bacteria↗