Characterization of the nodulation plasmid encoded chemoreceptor gene mcpG from Rhizobium leguminosarum

A putative methyl accepting chemotaxis protein gene (mcpG) of Rhizobium leguminosarum VF39SM was isolated and characterized. The gene was found to reside on the nodulation plasmid, pRleVF39d. The predicted mcpG ORF displayed motifs common to known methyl-accepting chemotaxis proteins, such as two transmembrane domains and high homology to the conserved methylation and signaling domains of well-characterized MCPs. Phenotypic analysis of mcpG mutants using swarm plates did not identify ligands for this putative receptor. Additionally, gene knockouts of mcpG did not affect a mutant strain's ability to compete for nodulation with the wild type. Notably, mcpG was found to be plasmid-encoded in all strains of R. leguminosarum and R. etli examined, though it was found on the nodulation plasmid only in a minority of strains.Based on sequence homology R. leguminosarum mcpG gene codes for a methyl accepting chemotaxis protein. The gene is plasmid localized in numerous Rhizobium spp. Although localized to the sym plasmid of VF39SM mcpG does not appear to participate in early nodulation events. A ligand for McpG remains to be found. Apparent McpG orthologs appear in a diverse range of proteobacteria. Identification and characterization of mcpG adds to the family of mcp genes already identified in this organism.Using a process termed chemotaxis, motile bacteria are capable of sensing their external environment and responding appropriately by moving towards increasing concentrations of nutrients and away from increasing concentrations of toxic compounds. The chemotaxis signaling pathway has been well characterized due to extensive studies using the model organism Escherichia coli, as well as detailed molecular characterization in numerous taxonomically distinct bacteria [1,2]. Methyl accepting chemotaxis proteins (MCPs) play key roles in the chemotactic response of many bacteria. These sensor proteins have N-terminal domains that detect attractants and repellents. When a ligand b