The CheYs of Rhodobacter sphaeroides

The Escherichia coli two-component chemosensory pathway has been extensively studied, and its response regulator, CheY, has become a paradigm for response regulators. However, unlike E. coli, most chemotactic nonenteric bacteria have multiple CheY homologues. The roles and cellular localization of the CheYs in Rhodobacter sphaeroides were determined. Only two CheYs were required for chemotaxis, CheY(6) and either CheY(3) or CheY(4). These CheYs were partially localized to either of the two chemotaxis signaling clusters, with the remaining protein delocalized. Interestingly, mutation of the CheY(6) phosphorylatable aspartate to asparagine produced a stopped motor, caused by phosphorylation on alternative site Ser-83 by CheA. Extensive mutagenesis of E. coli CheY has identified a number of activating mutations, which have been extrapolated to other response regulators (D13K, Y106W, and I95V). Analogous mutations in R. sphaeroides CheYs did not cause activation. These results suggest that although the R. sphaeroides and E. coli CheYs are similar in that they require phosphorylation for activation, they may differ in both the nature of the phosphorylation-induced conformational change and their subsequent interactions with the flagellar motor. Caution should therefore be used when projecting from E. coli CheY onto novel response regulators.