CHARACTERIZATION OF THE CHEMOTAXIS PROTEIN CHEW FROM RHODOBACTER-SPHAEROIDES AND ITS EFFECT ON THE BEHAVIOR OF ESCHERICHIA-COLI

In contrast to the situation in enteric bacteria, chemotaxis in Rhodob acter sphaeroides requires transport and partial metabolism of chemoat tractants. A chemotaxis operon has been identified containing homologu es of the enteric cheA, cheW, cheR genes and two homologues of the che Y gene. However, mutations in these genes have only minor effects on c hemotaxis. In enteric species, CheW transmits sensory information from the chemoreceptors to the histidine protein kinase, CheA. Expression of R. sphaeroides cheW in Escherichia coli showed concentration-depend ent inhibition of wild-type behaviour, increasing counter-clockwise ro tation and thus smooth swimming - a phenotype also seen when E. coli c heW is overexpressed in E.coli. In contrast, overexpression of R. spha eroides cheW in wild-type R. sphaeroides inhibited motility completely , the equivalent of inducing tumbly motility in E. coli. Expression of R. sphaeroides cheW in an E. coli Delta cheW chemotaxis mutant comple mented this mutation, confirming that CheW is involved in chemosensory signal transduction. However, unlike E. coli Delta cheW mutants, infr ame deletion of R. sphaeroides cheW did not affect either swimming beh aviour or chemotaxis to weak organic acids, although the responses to sugars were enhanced. Therefore, although CheW may act as a signal-tra nsduction protein in R. sphaeroides, it may have an unusual role in co ntrolling the rotation of the flagellar motor. Furthermore, the abilit y of a Delta cheW mutant to swim normally and show wild-type responses to weak acids supports the existence of additional chemosensory signa l-transduction pathways.