Modulation of Sinorhizobium meliloti quorum sensing by Hfq-mediated post-transcriptional regulation of ExpR.

In Sinorhizobium meliloti, the timing of quorum sensing (QS)-dependent gene expression is controlled at multiple levels. RNA binding protein Hfq contributes to the regulation of QS signal production, and this regulation is exerted both in the manner that involves the acyl homoserine lactone receptor ExpR, and via expR-independent mechanisms. In the expR+ strain of S. meliloti, deletion of hfq resulted in the hyper-accumulation of QS signals at low population densities, increased diversity of the QS signals in mid-to-late exponential phase and then led to a sharp decrease in QS signal accumulation in stationary phase. Quantitative polymerase chain reaction revealed that the accumulation of expR and sinI (but not sinR) mRNA was increased in the late exponential phase in an hfq-dependent manner. A translational, but not transcriptional, expR-uidA reporter was controlled by hfq, while both transcriptional and translational sinI-uidA reporters were regulated in the hfq-dependent manner. In co-immunoprecipation experiments, expR mRNA was bound to and then released from Hfq, similar to the positive controls (small regulatory RNA SmrC9, SmrC15, SmrC16 and SmrC45). Neither sinI nor sinR transcripts were detected in the pool of RNA heat-released from Hfq-RNA complexes. Therefore, post-transcriptional regulator Hfq controls the production and perception of QS signals, and at higher population densities this control is mediated directly via interactions with expR.