FIG 7.

Chemotaxis pathway responses mediated by Tar and McpS-Tar at different pH. (A) FRET measurements of the responses of E. coli strain VS181 expressing CheY-YFP/CheZ-CFP FRET pair and Tar as sole chemoreceptor, to a stepwise addition and subsequent removal of 100 μM L-Asp (indicated by down and up arrows, respectively) at different values of ambient pH (indicated with different colors). The ratio of YFP to CFP fluorescence represents the FRET signal and thus the activity of the chemotaxis pathway. (B) Response amplitudes of the VS181 strain carrying Tar (red) or McpS-Tar (purple) to their specific ligands, 100 μM L-Asp or 100 μM L-malate, respectively, as a function of pH, normalized to the response at pH 7.0. Dose dependence of responses mediated by Tar (C) and by McpS-Tar (D) at different values of ambient pH. The amplitudes of the initial FRET response were calculated from changes in the ratio of YFP/CFP fluorescence after stimulation with the indicated ligand concentrations and normalized to the saturated response. Error bars indicate the standard errors of three independent experiments; wherever they are invisible, error bars are smaller than the symbol size. Data were fitted using Hill equation.