The relationship between <i>pqs</i> gene expression and acylhomoserine lactone signaling in <i>Pseudomonas aeruginosa</i>

The opportunistic pathogen <i>Pseudomonas aeruginosa</i> has complex quorum sensing (QS) circuitry, which involves two acylhomoserine lactone (AHL) systems, the LasI AHL synthase and LasR AHL-dependent transcriptional activator system and the RhlI AHL synthase-RhlR AHL-responsive transcriptional activator. There is also a quinoline signaling system [the <i>Pseudomonas</i> quinolone signal (PQS) system]. Although there is a core set of genes regulated by the AHL circuits, there is strain-to-strain variation in the non-core QS regulon. A size reduction of the QS regulon occurs in laboratory evolution experiments with the model strain PAO1. We used transcriptomics to test the hypothesis that reductive evolution in the PAO1 QS regulon can in large part be explained by a null mutation in <i>pqsR</i>, the gene encoding the transcriptional activator of the <i>pqs</i> operon. We found that PqsR had very little influence on the AHL QS regulon. This was a surprising finding because the last gene in the PqsR-dependent <i>pqs</i> operon, <i>pqsE</i>, codes for a protein, which physically interacts with RhlR, and this interaction is required for RhlR-dependent activation of some genes. We used comparative transcriptomics to examine the influence of a <i>pqsE</i> mutation on the QS regulon and identified only three transcripts, which were strictly dependent on PqsE. By using reporter constructs, we showed that the PqsE influence on other genes was dependent on experimental conditions and we have gained some insight about those conditions. This work adds to our understanding of the plasticity of the <i>P. aeruginosa</i> QS regulon and to the role PqsE plays in RhlR-dependent gene activation.IMPORTANCEOver many generations of growth in certain conditions, <i>Pseudomonas aeruginosa</i> undergoes a large reductive evolution in the number of genes activated by quorum sensing. Here, we rule out one plausible route of the reductive evolution: that a mutation in a transcriptional activator PqsR or the PqsR activation of <i>pqsE</i>, which codes for a chaperone for the quorum sensing signal-responsive transcription factor RhlR, explains the finding. We further provide information about the influence of PqsR and PqsE on quorum sensing in <i>P. aeruginosa</i>.