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Co‐ordination of quorum‐sensing regulation in Rhizobium leguminosarum by induction of an anti‐repressor

Analysis of quorum‐sensing (QS) regulation in Rhizobium leguminosarum revealed an unusual type of gene regulation that relies on the population density‐dependent accumulation of an anti‐repressor. The cinS gene, which is co‐transcribed with the N‐acyl‐homoserine‐lactone synthase gene cinI, is required to fully induce rhiR and raiR, whose products, together with their partner AHL synthases, regulate other genes in a QS‐regulated hierarchy. Purified CinS bound to the R. leguminosarum transcriptional regulator PraR, which repressed rhiR and raiR expression. PraR bound to the rhiR and raiR promoters and CinS displaced PraR from these promoters, thereby inducing their expression. Although induction of cinS required CinI‐made AHL, it appears CinS does not require the AHL for its anti‐repressor function. The LuxR‐type regulator ExpR was also required for normal induction of rhiR and raiR and it appears that this occurs by ExpR repressing the transcription of praR. Therefore ExpR and CinS act independently to attenuate PraR action, ExpR by repressing its transcription and CinS by attenuating its repressive activity. Thus, as CinS accumulates in a population density‐dependent manner it induces the QS hierarchy by relieving PraR‐mediated repression of rhiR and raiR.