Analysis of a chemotaxis operon in Rhizobium meliloti

Genes controlling chemotaxis towards L-amino acids and D-mannitol in Rhizobium meliloti have been identified by Tn5 insertions that lead to chemotaxis-deficient mutants. The tagged genes span an 8.7 kbp region that has been sequenced. These genes are part of a large operon containing three novel open reading frames, orf1, orf2 and orf9, and six familiar chemotaxis (che) genes, cheY1-cheA-cheW-cheR-cheB-cheY2, that have been assigned by their similarity to known Escherichia coli genes. The second copy of cheY may be part of a second signalling chain; orf1 and orf2 encode sequence motifs that resemble the signalling domain of E. coli MCPs (methyl-accepting chemotaxis proteins), while the product of orf9 may contain a transmembrane domain. No protein methylation has been observed in Rhizobium meliloti in response to L-amino acids. However, the presence of cheR (methyltransferase gene) and cheB (methylesterase gene) suggested that MCPs are likely components of the chemotactic response in R. meliloti. Therefore, it is postulated that two chemotaxis pathways are functional in R. meliloti: one responds to L-amino acids via ORF1-ORF2, whereas the other (probably responding to specific plant exudates) acts via MCP-like receptors, and both interact with the central components CheW-CheA-CheY1 and/or CheY2.