Quorum-Sensing Receptor-Ligand Interactions in Pseudomonas aeruginosa

Abstract

Quorum sensing is a cell-to-cell communication mechanism that bacteria use to orchestrate collective behaviors. Quorum sensing relies on the production, release, accumulation, and detection of extracellular signal molecules called autoinducers. The opportunistic pathogen Pseudomonas aeruginosa possesses multiple quorum-sensing systems, each composed of a receptor-autoinducer pair. Two of the P. aeruginosa quorum-sensing receptors, LasR and RhlR, are canonical LuxR-type receptors, meaning that they closely resemble the founding quorum-sensing receptor, as well as quorum-sensing receptors present in other species of Gram-negative bacteria. The cognate autoinducers for LuxR-type receptors are homoserine lactones. When bound to their ligands, LuxR-type receptors function as transcription factors that activate hundreds of genes underpinning collective behaviors. This work explores LasR and RhlR ligand binding preferences to understand how each of these receptors interacts with cognate and non-cognate ligands and how receptor-ligand interactions drive regulation of downstream quorum-sensing products. First, the work shows how LasR distinguishes between similar homoserine lactones as well as how LasR preferentially selects its own ligand from a mixture of related molecules. Second, the work reports RhlR ligand-independent mutants that display constitutive activity in vivo, providing insight into the enigmatic RhlR structure. Furthermore, the research demonstrates how the activities of RhlR ligand-independent mutants are kept in check with respect to regulation of quorum-sensing products. Finally, this thesis uses a RhlR ligand-independent mutant to reveal how RhlR works in conjunction with the PqsE thioesterase to regulate the production of the critical virulence factor pyocyanin.