A widely conserved molecular switch controls quorum sensing and symbiosis island transfer in M esorhizobium loti through expression of a novel antiactivator

Summary ICE Ml Sym R7A of M esorhizobium loti is an integrative and conjugative element ( ICE ) that confers the ability to form a nitrogen‐fixing symbiosis with L otus species. Horizontal transfer is activated by TraR and N ‐acyl‐homoserine lactone ( AHL ), which can stimulate ICE excision in 100% of cells. However, in wild‐type cultures, the ICE is excised at low frequency. Here we show that QseM , a widely conserved ICE ‐encoded protein, is an antiactivator of TraR . Mutation of qseM resulted in TraR ‐dependent activation of AHL production and excision, but did not affect transcription of traR . QseM and TraR directly interacted in a bacterial two‐hybrid assay in the presence of AHL . qseM expression was repressed by a DNA ‐binding protein QseC , which also activated qseC expression from a leaderless transcript. QseC differentially bound two adjacent operator sites, the lower affinity of which overlapped the −35 regions of the divergent qseC ‐ qseM promoters. QseC homologues were identified on ICE s, TraR / TraM ‐regulated plasmids and restriction‐modification cassettes, suggesting a conserved mode of regulation. Six QseC variants with distinct operators were identified that showed evidence of reassortment between mobile elements. We propose that QseC and QseM comprise a bimodal switch that restricts quorum sensing and ICE Ml Sym R7A transfer to a small proportion of cells in the population.