RopB represses the transcription of in the absence of SIP in group A .

RopB is a quorum-sensing regulator that binds to the SpeB-inducing peptide (SIP) under acidic conditions. SIP is known to be degraded by the endopeptidase PepO, whose transcription is repressed by the CovR/CovS two-component regulatory system. Both SIP-bound RopB (RopB-SIP) and SIP-free RopB (apo-RopB) can bind to the promoter; however, only RopB-SIP activates transcription. In this study, we found that the SpeB expression was higher in the mutant than in the SIP-inactivated (*) mutant. Furthermore, the deletion of in the * mutant derepressed expression, suggesting that apo-RopB is a transcriptional repressor of Up-regulation of PepO in the mutant degraded SIP, resulting in the down-regulation of We demonstrate that deleting in the mutant derepressed the expression, suggesting that the repression in this mutant was mediated not only by PepO-dependent SIP degradation but also by apo-RopB. These findings reveal a crosstalk between the CovR/CovS and RopB-SIP systems and redefine the role of RopB in regulating expression in group A .