SummaryIn contrast to the situation in enteric bacteria, chemotaxis in Rhodobacter sphaeroides requires transport and partial metabolism of chemoattractants. A chemotaxis operon has been identified containing homologues of the enteric cheA, cheW, cheR genes and two homologues of the cheY gene. However, mutations in these genes have only minor effects on chemotaxis. 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 concentrationdependent inhibition of wild-type behaviour, increasing counter-clockwise rotation and thus smooth swimming -a phenotype also seen when E. coli cheW is overexpressed in E. coli. In contrast, overexpression of R. sphaeroides 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 ⌬cheW chemotaxis mutant complemented this mutation, confirming that CheW is involved in chemosensory signal transduction. However, unlike E. coli ⌬cheW mutants, inframe deletion of R. sphaeroides cheW did not affect either swimming behaviour or chemotaxis to weak organic acids, although the responses to sugars were enhanced. Therefore, although CheW may act as a signal-transduction protein in R. sphaeroides, it may have an unusual role in controlling the rotation of the flagellar motor. Furthermore, the ability of a ⌬cheW mutant to swim normally and show wild-type responses to weak acids supports the existence of additional chemosensory signal-transduction pathways.