PqsE adapts the activity of the <i>Pseudomonas aeruginosa</i> quorum-sensing transcription factor RhlR to both autoinducer concentration and promoter sequence identity

<i>Pseudomonas aeruginosa</i> is an opportunistic human pathogen that poses a significant health threat. Many pathogenic behaviors of <i>P. aeruginosa</i> are under control of the bacterial cell-cell communication system known as quorum sensing (QS). One of the QS master regulators, RhlR, is a receptor/transcription factor that not only relies on binding of its canonical ligand, <i>N</i>-butyrylhomoserine lactone (C4-HSL), but additionally requires a protein-protein interaction with the enzyme, PqsE. We constructed heterologous reporter strains in <i>Escherichia coli</i> that allow measurements of the reliance of RhlR on C4-HSL and/or PqsE binding for the ability to activate transcription of three RhlR-regulated genes: <i>rhlA</i> (PqsE independent), <i>phzM</i> (PqsE dependent), and <i>azeB</i> (PqsE inhibited). Analogous assays measuring activation of the three genes in <i>P. aeruginosa</i> were performed, and the patterns observed correlated tightly with the heterologous reporter assays. These results confirm that the binding of PqsE to RhlR is able to fine-tune RhlR transcription factor activity in a promoter-specific manner and prove that this ability is independent of other factors present in <i>P. aeruginosa</i>.IMPORTANCE<i>Pseudomonas aeruginosa</i> is an opportunistic human pathogen that can cause fatal infections. There exists an urgent need for new, effective antimicrobial agents to combat <i>P. aeruginosa</i>. The PqsE-RhlR protein-protein interaction is essential for <i>P. aeruginosa</i> to be able to make toxins, form biofilms, and infect host organisms. In this study, we use both non-native models in <i>Escherichia coli</i> and measurements of gene expression/toxin production in <i>P. aeruginosa</i> to show that the PqsE-RhlR interaction enables fine-tuned gene expression and a heightened ability of <i>P. aeruginosa</i> to adapt to external conditions. These findings will be highly valuable as continued efforts are made to design inhibitors of the PqsE-RhlR interaction and test them as potential antimicrobial agents against <i>P. aeruginosa</i> infections.