Identification and localization of a methyl-accepting chemotaxis protein in Rhodobacter sphaeroides

Genes coding for a classical membrane spanning chemoreceptor (mcpG) and a r esponse regulator (cheY4) were identified in a region of Rhodobacter sphaer oides DNA unlinked to either of the two previously identified chemosensory operons. Immunogold electron microscopy had shown that the expression of ch emoreceptors in R. sphaeroides varies with growth conditions. Using GFP fus ed to the newly identified McpG, we examined the targeting of this single m ethyl-accepting chemotaxis protein (MCP) under different growth conditions. The gene encoding the C-terminal McpG-GFP fusion was introduced by homolog ous recombination into the chromosome, replacing the wild-type gene. The re sultant protein localized to the poles of the cell under aerobic, photohete rotrophic and anaerobic dark conditions, demonstrating that this MCP is exp ressed under all three growth conditions. More protein was always found at one pole than the other. The polar fluorescence increased during the cell c ycle, with protein becoming evident at the second pole around the time of s eptation. At division, each daughter cell had a label at one pole, but the intensity of fluorescence was higher in the daughter cell containing the or iginal labelled pole. McpG localization was not altered in a che Operon 1 d eletion strain, lacking CheW1 and CheA1, but a che Operon 2 deletion strain , lacking CheW2, CheW3 and CheA2, showed significantly reduced polar locali zation. This observation indicates that polar localization of McpG depends on Che proteins encoded by Operon 2, but not homologues encoded by Operon 1 .