Modulation of the quorum sensing cascade by MexT-regulated factors.

uses quorum sensing (QS), a cell-cell communication system that enables it to sense cell density and to alter gene expression. has three complete QS circuits controlled by the regulators LasR, RhlR, and PqsR, that together activate hundreds of genes. In the well-studied strain PAO1, QS is organized hierarchically, with PqsR and RhlR activity dependent on LasR. This hierarchy depends on the non-QS transcription factor MexT; deletion of allows for RhlR activity in the absence of LasR. We aimed to identify how MexT modulates the QS architecture. We compared the transcriptome of PAO1 to that of PAO1Δ and determined a MexT regulon. We identified two MexT-regulated operons that may affect the QS hierarchy: the efflux pump genes - and the quinolone signal (PQS) synthesis genes . We tested whether the products of these genes affected the QS hierarchy. A knockout mutant, like a deletion mutant, exhibited RhlR activity earlier, and to a higher magnitude, than wild-type PAO1. MexEF-OprN is known to export quinolones, and we found that exogenous addition of PQS, through PqsE, also resulted in earlier and higher magnitude of RhlR activity compared to wild-type PAO1. We also discovered alternate QS architectures in clinical isolates, where RhlR activity is not fully dependent on LasR. In these isolates, surprisingly, MexT does not influence the relationship between LasR and RhlR. Our work reveals a new suite of factors that regulate QS in , with implications for bacterial behaviors in environmental and clinical settings.