TYROSINE-106 OF CHEY PLAYS AN IMPORTANT ROLE IN CHEMOTAXIS SIGNAL-TRANSDUCTION IN ESCHERICHIA-COLI

CheY is the response regulator in the signal transduction pathway of b acterial chemotaxis. Position 106 of CheY is occupied by a conserved a romatic residue (tyrosine or phenylalanine) in the response regulator superfamily, A number of substitutions at position 106 have been made and characterized by both behavioral and biochemical studies, On the b asis of the behavioral studies, the phenotypes of the mutants at posit ion 106 can be divided into three;categories: (i) hyperactivity, with a tyrosine-to-tryptophan mutation (Y106W) causing increased tumble sig naling but impairing chemotaxis; (ii) low-level activity, with a tyro sine-to-phenylalanine change (Y106F) resulting in decreased tumble sig naling acid chemotaxis; and (iii) no activity, with substitutions such as Y106L, Y106I, Y106V, Y106G, and Y106C resulting in no chemotaxis a nd a smooth-swimming phenotype. All three types of mutants can be phos phorylated by CheA-phosphate in vitro to a level similar to that of wi ld-type CheY. Autodephosphorylation rates are similar for all categori es of mutants, All mutant proteins displayed less than twofold increas ed rates compared with wild-type CheY. Binding of the mutant proteins to FliM was similar to that of the wild-type CheY in the CheY-FliM bin ding assays. The combined results from in vivo behavioral and in vitro biochemical studies suggest that the diverse phenotypes of the Y106 m utants are not due to a variation in phosphorylation or dephosphorylat ion ability nor in affinity for the switch. With reference to the stru ctures of wild-type CheY and the T871 CheY mutant, our results suggest that rearrangements of the orientation of the tyrosine side chain at position 106 are involved in the signal transduction of Chef: These da ta also suggest that the binding of phosphoryl-CheY to the flagellar m otor is a necessary, but not sufficient, event for signal transduction .