The roles of the multiple CheW and CheA homologues in chemotaxis and in chemoreceptor localization in Rhodobacter sphaeroides
Rhodobacter sphaeroides has multiple homologues of most of the Escherichia coli chemotaxis genes, organized in two major operons and other, unlinked, loci. These include cheA 1 and cheW 1 (che Op 1) and cheA 2 cheW 2 and cheW 3 (che Op 2). We have deleted each of these cheA and cheW homologues in-frame and examined the chemosensory behaviour of these strains on swarm plates and in tethered cell assays. In addition, we have examined the effect of these deletions on the polar localization of the chemoreceptor McpG. In E. coli, deletion of either cheA or cheW results in a non-chemotactic phenotype, and these strains also show no receptor clustering. Here, we demonstrate that CheW 2 and CheA 2 are required for the normal localization of McpG and for normal chemotactic responses under both aerobic and photoheterotrophic conditions. Under aerobic conditions, deletion of cheW 3 has no significant effect on McpG localization and only has an effect on chemotaxis to shallow gradients in swarm plates. Under photoheterotrophic conditions, however, CheW 3 is required for McpG localization and also for chemotaxis both on swarm plates and in the tethered cell assay. These phenotypes are not a direct result of delocalization of McpG, as this chemoreceptor does not mediate chemotaxis to any of the compounds tested and can therefore be considered a marker for general methyl-accepting chemotaxis protein (MCP) clustering. Thus, there is a correlation between the normal localization of McpG (and presumably other chemoreceptors) and chemotaxis. We propose a model in which the multiple different MCPs in R. sphaeroides are contained within a polar chemoreceptor cluster. Deletion of cheW 2 and cheA 2 under both aerobic and photoheterotrophic conditions, and cheW 3 under photoheterotrophic conditions, disrupts the cluster and hence reduces chemotaxis to any compound sensed by these MCPs.