CHEMOTACTIC RESPONSE REGULATOR MUTANT CHEY95IV EXHIBITS ENHANCED BINDING TO THE FLAGELLAR SWITCH AND PHOSPHORYLATION-DEPENDENT CONSTITUTIVESIGNALING

CheY, a response regulator protein in bacterial chemotaxis, mediates s wimming behaviour through interaction with the flagellar switch protei n, FliM. In its active, phosphorylated state, CheY binds to the motor switch complex and induces a change from counterclockwise (CCW) to clo ckwise (CW) flagellar rotation. The conformation of a conserved aromat ic residue, tyrosine 106, has been proposed to play an important role in this signalling process. Here, we show that an isoleucine to valine substitution in CheY at position 95 - in close proximity to residue 1 06 - results in an extremely CW, hyperactive phenotype that is depende nt on phosphorylation. Further biochemical characterization of this mu tant protein revealed phosphorylation and dephosphorylation rates that were indistinguishable from those of wild-type CheY. CheY9SIV, howeve r, exhibited an increased binding affinity to FliM. Taken together, th ese results show for the first time a correlation between enhanced swi tch binding and constitutive signalling in bacterial chemotaxis. Consi dering present structural information, we also propose possible models for the role of residue 95 in the mechanism of CheY signal transducti on.