Chemical Probes that Target a Dissociative LuxR-Type Quorum Sensing Receptor in Gram-Negative Bacteria

Quorum sensing (QS) allows bacteria to respond to changes in cell density and participate in collective behaviors. Interfering with QS could provide a strategy to block pathogenicity, reduce biofouling, and support biotechnology. Many common Gram-negative bacteria use LuxR-type QS receptors that regulate gene transcription in response to N-acyl l-homoserine lactone (AHL) signals. The most-studied LuxR-type receptors operate via an associative mechanism, i.e., they dimerize and associate with DNA upon ligand binding. In contrast, members of the less-studied class of dissociative LuxR-type receptors bind DNA as dimers in the absence of a ligand and dissociate from DNA upon ligand binding. Few chemical tools to modulate dissociative receptor activity are known. Such probes could provide new entry into mechanistic studies of LuxI/LuxR-type QS in general. In this report, we describe the discovery of synthetic modulators of EsaR, a dissociative LuxR-type receptor present in the plant pathogen Pantoea stewartii, based on AHL scaffolds. Compound activity was evaluated using both cell-based EsaR reporters and a phenotypic assay. We identified compound features associated with agonistic activity in EsaR, some of which were comparable to those of synthetic ligands active in other LuxR-type receptors. However, in contrast to prior studies of AHL mimics, no antagonists were uncovered in EsaR. These results provide chemical strategies to start to investigate mechanisms of ligand response in EsaR and define receptor features driving dissociative vs associative mechanisms in the LuxR-type receptor family. Our findings also suggest that alternate approaches may be required to develop competitive antagonists for dissociative LuxR-type receptors.