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 cheA1 and cheR1 (che Op1) and cheA2, cheR2, and cheB1 (che OP2). In-frame deletions of these cheR and cheB homologues were constructed and the chemosensory behaviour of the resultant mutants examined on swarm plates and in tethered cell assays. Under the conditions tested, CheR2 and CheB1 were essential for normal chemotaxis, whereas CheR1 was not. cheR2 and cheB1, but not cheR1, were also able to complement the equivalent E. coli mutants. However, none of the proteins were required for the correct polar localization of the chemoreceptor McpG in R. sphaeroides. 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 identified, although 2 of the 13 putative chemoreceptors, McpA and TlpT, do have similar sequences. This suggests that CheR2 either interacts with the NWETF motif of E. coli methyl-accepting chemotaxis proteins (MCPs), even though its native motif may be slightly different, or with another conserved region of the MCPs. Methanol release measurements show that R. sphaeroides has an adaptation system that is different from that of Bacillus subtilis and E. coli, with methanol release measurable on the addition of attractant but not on its removal. Intriguingly, CheA2, but not CheA1, is able to phosphorylate CheB1, suggesting that signaling through CheA1 cannot initiate feedback receptor adaptation via CheB1-P.