Deciphering the Symbiotic Significance of Quorum Sensing Systems of Sinorhizobium fredii HH103

Abstract

Quorum sensing (QS) is a bacterial cell-to-cell signaling mechanism that collectively regulates and synchronizes behaviors by means of small diffusible chemical molecules. In rhizobia, QS systems usually relies on the synthesis and detection of <i>N</i>-acyl-homoserine lactones (AHLs). In the model bacterium <i>Sinorhizobium meliloti</i> functions regulated by the QS systems TraI-TraR and SinI-SinR(-ExpR) include plasmid transfer, production of surface polysaccharides, motility, growth rate and nodulation. These systems are also present in other bacteria of the <i>Sinorhizobium</i> genus, with variations at the species and strain level. In <i>Sinorhizobium fredii</i> NGR234 phenotypes regulated by QS are plasmid transfer, growth rate, sedimentation, motility, biofilm formation, EPS production and copy number of the symbiotic plasmid (pSym). The analysis of the <i>S. fredii</i> HH103 genomes reveal also the presence of both QS systems. In this manuscript we characterized the QS systems of <i>S. fredii</i> HH103, determining that both TraI and SinI AHL-synthases proteins are responsible of the production of short- and long-chain AHLs, respectively, at very low and not physiological concentrations. Interestingly, the main HH103 <i>luxR</i>-type genes, <i>expR</i> and <i>traR</i>, are split into two ORFs, suggesting that in <i>S. fredii</i> HH103 the corresponding carboxy-terminal proteins, which contain the DNA-binding motives, may control target genes in an AHL-independent manner. The presence of a split <i>traR</i> gene is common in other <i>S. fredii</i> strains.