Polar Localization of CheA 2 in Rhodobacter sphaeroides Requires Specific Che Homologs

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Abstract
Rhodobacter sphaeroides is a motile bacterium that has multiple chemotaxis genes organized predominantly in three major operons (cheOp1, cheOp2, and cheOp3). The chemoreceptor proteins are clustered at two distinct locations, the cell poles and in one or more cytoplasmic clusters. One intriguing possibility is that the physically distinct chemoreceptor clusters are each composed of a defined subset of specific chemotaxis proteins, including the chemoreceptors themselves plus specific CheW and CheA proteins. Here we report the subcellular localization of one such protein, CheA2, under aerobic and photoheterotrophic growth conditions. CheA2 is predominantly clustered and localized at the cell poles under both growth conditions. Furthermore, its localization is dependent upon one or more genes in cheOp2 but not those of cheOp1 or cheOp3. In E. coli, the polar localization of CheA depends upon CheW. The R. sphaeroides cheOp2 contains two cheW genes. Interestingly, CheW2 is required under both aerobic and photoheterotrophic conditions, whereas CheW3 is not required under aerobic conditions but appears to play a modest role under photoheterotrophic conditions. This suggests that R. sphaeroides contains at least two distinct chemotaxis complexes, possibly composed of proteins dedicated for each subcellular location. Furthermore, the composition of these spatially distinct complexes may change under different growth conditions.