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) an d cheW(3) (che Op(2)). We have deleted each of these cheA and cheW homologu es in-frame and examined the chemosensory behaviour of these strains on swa rm plates and in tethered cell assays. In addition, we have examined the ef fect 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 dem onstrate that CheW(2) and CheA(2) are required for the normal localization of McpG and for normal chemotactic responses under both aerobic and photohe terotrophic conditions. Under aerobic conditions, deletion of cheW(3) has n o significant effect on McpG localization and only has an effect on chemota xis to shallow gradients in swarm plates. Under photoheterotrophic conditio ns, however, CheW(3) is required for McpG localization and also for chemota xis both on swarm plates and in the tethered cell assay. These phenotypes a re not a direct result of delocalization of McpG, as this chemoreceptor doe s not mediate chemotaxis to any of the compounds tested and can therefore b e considered a marker for general methyl-accepting chemotaxis protein (MCP) clustering. Thus, there is a correlation between the normal localization o f McpG (and presumably other chemoreceptors) and chemotaxis. We propose a m odel in which the multiple different MCPs in R. sphaeroides are contained w ithin a polar chemoreceptor cluster. Deletion of cheW(2) and cheA(2) under both aerobic and photoheterotrophic conditions, and cheW(3) under photohete rotrophic conditions, disrupts the cluster and hence reduces chemotaxis to any compound sensed by these MCPs.