CerM and Its Antagonist CerN Are New Components of the Quorum Sensing System in <i>Cereibacter sphaeroides</i>, Signaling to the CckA/ChpT/CtrA System

Cereibacter sphaeroides has a quorum sensing (QS) system that has been partially characterized. Using a bioinformatic approach, six LuxR homologs and one homolog of the acylhomoserine lactone synthase were identified in this bacterium, including the previously characterized CerR and CerI proteins. This study focused on determining the roles of two LuxR homologs, CerM and CerN. CerN lacks the HTH domain and, together with CerM, controls the expression of ctrA, which is part of the TCS CckA/ChpT/CtrA. CtrA is widely conserved in alpha-proteobacteria and regulates flagellar motility and other cellular processes. Genetic and biochemical data suggest that CerM indirectly represses ctrA expression, which is counteracted by its interaction with CerN-AHL. A transcriptomic study identified 181 genes regulated by CerM/CerN, with a conserved sequence in their regulatory regions likely indicating the CerM binding site. This hypothesis was supported by in vitro and in vivo DNA-protein interaction assays. Our results identified a transcription factor that could connect the QS system with the regulation of the two-component system CckA/ChpT/CtrA.