CheR- and CheB-dependent chemosensory adaptation system of Rhodobacter sphaeroides

Rhodobacter sphaeroides has multiple homologues of most of the Escherichia coli chemotaxis genes, organized in three major operons and other, unlinked , loci. These include cheA(1) and cheR(1) (che Op(1)) and cheA(2) cheR(2) a nd cheB(1) (che Op(2)). In-frame deletions of these cheR and cheB homologue s were constructed and the chemosensory behaviour of the resultant mutants examined on swarm plates and in tethered cell assays. Under the conditions tested, CheR(2) and CheB(1) were essential for normal chemotaxis, whereas C heR(1) was not cheR(2) and cheB(1) but not cheR(2) were also able to comple ment the equivalent E. coli mutants. However, none of the proteins were req uired for the correct polar localization of the chemoreceptor McpG in R. sp haeroides. In E. coli, CheR binds to the NWETF motif on the high-abundance receptors, allowing methylation of both high- and low-abundance receptors. This motif is not contained on any R. sphaeroides chemoreceptors thus far i dentified, although 2 of the 13 putative chemoreceptors, McpA and TlpT, do have similar sequences. This suggests that CheR(2) either interacts with th e NWETF motif of E. coli methyl-accepting chemotaxis proteins (MCPs), even though its native motif may be slightly different, or with another conserve d region of the MCPs. Methanol release measurements show that R. sphaeroide s has an adaptation system that is different from that of Bacillus subtilis and E. coli, with methanol release measurable on the addition of attractan t but not on its removal. Intriguingly, CheA(2), but not CheA(1) is able to phosphorylate CheB(1), suggesting that signaling through CheA(1) cannot in itiate feedback receptor adaptation via CheB(1)-P.