Identification of two quorum-sensing systems in Sinorhizobium meliloti

Sinorhizobium meliloti is a free-living soil bacterium which is capable of establishing a symbiotic relationship with the alfalfa plant (Medicago sativa). This symbiosis involves a network of bacterium-host signaling, as well as the potential for bacterium-bacterium communication, such as quorum sensing. In this study, we characterized the production of N-acyl homoserine lactones (AHLs) by two commonly used S. meliloti strains, AK631 and Rm1021. We found that AK631 produces at least nine different AHLs, while Rm1021 produces only a subset of these molecules. To address the difference in AHL patterns between the strains, we developed a novel screening method to identify the genes affecting AHL synthesis. With this screening method, chromosomal groEL (groELc) was shown to be required for synthesis of the AHLs that are unique to AK631 but not for synthesis of the AHLs that are made by both AK631 and Rm1021. We then used the screening procedure to identify a mutation in a gene homologous to traM of Agrobacterium tumefaciens, which was able to suppress the phenotype of the groELc mutation. A traR homolog was identified immediately upstream of traM, and we propose that its gene product requires a functional groELc for activity and is also responsible for inducing the synthesis of the AHLs that are unique to AK631. We show that the traR/traM locus is part of a quorum-sensing system unique to AK631 and propose that this locus is involved in regulating conjugal plasmid transfer. We also present evidence for the existence of a second quorum-sensing system, sinR/sinI, which is present in both AK631 and Rm1021.