Evolution of PqsE as a Pseudomonas aeruginosa -specific regulator of LuxR-type receptors: insights from Pseudomonas and Burkholderia

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that poses a significant public health threat, particularly in healthcare settings. A key determinant of P. aeruginosa virulence is the regulated synthesis and release of extracellular products, which is controlled by a cell density-dependent signaling system known as quorum sensing (QS). P. aeruginosa uses a complex QS network, including two systems that rely on diffusible N-acylhomoserine lactone (AHL) signal molecules. The LuxR-type receptor RhlR is unique in that it requires not only its cognate AHL but also the accessory protein PqsE to maximally bind to promoter DNA and to initiate transcription. Our group demonstrated that PqsE physically interacts with RhlR, enhancing its affinity for target promoters across the P. aeruginosa genome. Although LuxR-type receptors are widespread in Gram-negative bacteria and important for pathogenesis, PqsE orthologs are restricted to Pseudomonas and Burkholderia species. This study explored the conservation of PqsE and examined PqsE ortholog structure-function across different species. Our results show that PqsE in Pseudomonas retain their functional interactions with RhlR homologs, unlike PqsE orthologs in Burkholderia spp., which do not interact with their respective LuxR-type receptors. Additionally, we assessed the AHL preferences of different receptors and hypothesized that the PqsE-RhlR interaction evolved to stabilize the inherently unstable RhlR, preventing its degradation. Indeed, we observe higher levels of RhlR protein turnover in a strain lacking pqsE compared to WT, which can be rescued in a strain lacking the Lon protease.