CHEMOTACTIC SIGNAL INTEGRATION IN BACTERIA

Chemotactic signaling in Escherichia coil involves transmission of bot h negative and positive signals, In order to examine mechanisms of sig nal processing, behavioral responses to dual inputs have been measured by using photoactivatable ''caged'' compounds, computer video analysi s, and chemoreceptor deletion mutants, Signaling from Tar and Tsr, two receptors that sense amino acids and pH, was studied, In a Tar deleti on mutant the photoactivated release of protons, a Tsr repellent, and of serine, a Tsr attractant, in separate experiments Pt pH 7.0 resulte d in tumbling (negative) or smooth-swimming (positive) responses in ca . 50 and 140 ms, respectively, Simultaneous photorelease of protons an d serine resulted in a single tumbling or smooth-swimming response, de pending on the relative amounts of the two effecters, In contrast, in wild-type E. coil, proton release at pH 7.0 resulted in a biphasic res ponse that was attributed to Tsr-mediated tumbling followed by Tar-med iated smooth-swimming, In wild-type E. coli at more alkaline pH values the Tar-mediated signal was stronger than the Tsr signal, resulting i n a strong smooth-swimming response preceded by a diminished tumbling response, These observations imply that (i) a single receptor time-ave rages the binding of different chemotactic ligands generating a single response; (ii) ligand binding to different receptors can result in a nonintegrated response with the tumbling response preceding the smooth -swimming response; (iii) however, chemotactic signals of different in tensities derived from different receptors can also result in an appar ently integrated response; and (iv) the different chemotactic response s to protons at neutral and alkaline pH may contribute to E. coli migr ation toward neutrality.