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The key control step in E. coli chemotaxis is regulation of CheA kinase activity by a set of four transmembrane chemoreceptors. The receptor dimers can form trimeric complexes (trimers of dimers), and these trimers can be joined by a bridge thought to consist of a CheW monomer, a CheA dimer, and a second CheW monomer. It has been proposed that trimers of receptor dimers may be joined by CheW-CheA dimer-CheW links to form an extended hexagonal lattice that may be the structural basis of the chemoreceptor patches seen in E. coli. The receptor/CheA/CheW ternary complex is a membrane-spanning allosteric enzyme whose activity is regulated by protein interactions. The study presented in this dissertation investigated intermolecular and intramolecular interactions that affect the chemotactic signal processing. I have examined functional interactions between the serine receptor Tsr and the aspartate receptor Tar using a receptor coupled in vitro phosphorylation assay. The results reveal the emergent properties of mixed receptor populations and emphasize their importance in the integrated signal processing that underlies bacterial chemotaxis. A mutational analysis of the extreme C-terminus (last fifty residues) of Tar is also presented. The results implicate the receptor C-terminus in maintenance of baseline receptor activity and in attractant-induced transmembrane signaling. They also suggest how adaptive methylation might counteract the effects of attractant binding.

Chez Cupriavidus metallidurans CH34, le complexe CnrYXH contribue à réguler l'expression des gènes de régulation et de résistance au cobalt et au nickel en fonction de la concentration environnementale de ces cations. La fixation de cobalt ou de nickel sur le domaine périplasmique senseur de CnrX induit des modifications conformationnelles à l'origine de la transduction du signal transmembranaire. Cela conduit à rendre le facteur sigma CnrH disponible pour sa liaison à l'ARN polymérase (ARNP) dans le cytoplasme. Le complexe CnrH:ARNP se fixe alors spécifiquement sur les promoteurs des gènes cnrY et cnrC pour initier la transcription des gènes de régulation (cnrYXH) et de résistance (cnrCBAT). Dans le but de déterminer la nature de ce signal, mon projet de thèse visait à cartographier les interactions protéine:protéine au sein du complexe CnrYXH dans les trois compartiments concernés : le périplasme, la membrane plasmique et le cytoplasme. La documentation des déterminants d'interaction entre CnrX, CnrY et CnrH a permis d'élaborer un modèle structural et fonctionnel pour le complexe CnrYXH et ses homologues soulevant des hypothèses nouvelles sur le fonctionnement du système Cnr. / The CnrYXH complex contributes to regulate the expression of the regulatory genes and resistance genes involved in cobalt and nickel resistance in Cupriavidus metallidurans CH34. The binding of nickel or cobalt to CnrX in the periplasm induces conformational modifications that could trigger transmembrane signal transduction. As a result, CnrH is made available in cytoplasm for binding to RNA polymerase. The CnrH:RNA polymerase complex binds specifically the cnr promoters and initiates transcription of both the regulatory genes (cnrYXH) and resistance genes (cnrCBAT). In order to delineate the mechanism of signal transduction through the membrane, I studied the interactions between the three partners in the different cellular compartments: periplasm, plasmic membrane and cytoplasm. The identification of the interaction determinants between CnrX, CnrY and CnrH allowed us to propose a structural and functional model for the CnrYXH complex and its homologues. This model brings up new hypothesis on the function of Cnr system.