Binding of the Escherichia coli response regulator CheY to its target measured in vivo by fluorescence resonance energy transfer

In Escherichia coli chemotaxis, signaling depends on modulation of the level of phosphorylation of CheY, a small protein that couples receptors and flagellar motors. Working in vivo, we used fluorescence resonance energy transfer (FRET) to measure the interaction of CheY∼P with its target, FliM. Binding of CheY∼P to FliM was found to be much less cooperative than motor switching; however, under the conditions of our experiment, most of the FliM appeared to be in the cytoplasm. We studied signal processing times in the chemotaxis pathway by measuring the changes in CheY∼P binding to FliM on flash release of caged chemoeffectors. Following sudden addition of attractant, the amount of CheY∼P bound to FliM decayed exponentially with a rate constant of about 2 s(−1). Following sudden addition of repellent, FliM occupancy increased with a rate constant of about 20 s(−1). Using these data, we were able to construct a simple model for the chemotactic pathway and to estimate values of rate constants for several key reactions.