Signal transduction and adaptation by polymodal receptors

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The chemotaxis in Escherichia coli and Salmonella typhimurium has been serving as a model system to study signal transduction pathways at the molecular level. The transmembrane chemoreceptors sense environmental stimuli, including chemoattractants , chemorepellents, pH and temperature, and transmit signals to the cytoplasmic histidine kinase CheA. Persisting stimulation causes adaptation via methylation/demethylation of the relevant chemoreceptor. In this study, we have focused on polymodal sensing of the chemoreceptors and the mechanisms of adaptation through interaction between a receptor and the methyltransferase CheR. We obtained various important results concerning the following topics that provide new insights into the mechanisms underlying the chemotactic signal transduction : (i) the recognition of citrate by the chemoreceptor Tcp (published) ; (ii) the involvement of the "linker" region in the recognition of pH by the chemoreceptors, Tar and Tsr (published) ; (iii) the involvement of the transmembrane regions in thermosensing by the chemoreceptors, Tar and Tap (in preparation) ; (iv) role of the C-terminal pentapeptide sequence of the chemoreceptors in the binding of CheR (published ; submitted) ; (v) dual recognition of the bacterial chemoreceptor Tar by chemotaxis-specific domains of the CheR methyltransferase (submitted) ; (vi) the effect of methylation of the chemoreceptor Tar on its ligand-binding affinity (in preparation) ; and (vii) the roles of the three cheA homologs in chemotaxis of Vibrio cholerae (published).