PqsE acts as an adaptor protein for the quorum-sensing transcription factor RhlR in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a human pathogen that poses a significant health threat. Pathogenic behaviors of P. aeruginosa 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, C4-homoserine lactone (HSL), but additionally requires a protein-protein interaction with the enzyme, PqsE. We constructed heterologous reporter strains in Escherichia coli 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: rhlA (PqsE-independent), phzM (PqsE-dependent), and azeB (PqsE-suppressed). Analogous assays measuring activation of the three genes in P. aeruginosa were performed and the patterns observed correlated tightly with the heterologous reporter assays. These results indicate that binding of PqsE to RhlR is able to fine-tune RhlR transcription factor activity in a promoter-specific manner.