Insights into the Mechanisms of Small Molecule-Induced Agonism and Antagonism of a Quorum Sensing Receptor in Pseudomonas aeruginosa

Common bacteria use chemical signals to assess their local cell density in a process called quorum sensing (QS). The pathogen Pseudomonas aeruginosa utilizes QS to control many virulence factors and traits involved in infection. Two QS signals used by P. aeruginosa are N-acyl L-homoserine lactones, which are recognized by three LuxR-type receptors, LasR, QscR, and RhlR, to alter gene expression. Small molecule-based modulation of LuxR-type receptors has been the focus of extensive efforts to attenuate virulence in P. aeruginosa. However, further development of these compounds has been slowed by the limited understanding of their mechanisms of action, either as agonists or antagonists. Specifically, how binding of ligand effectively transduces information to the DNA binding domain of LuxR-type proteins to promote or inhibit DNA binding is largely unknown. Herein, we report a suite of biochemical and biophysical experiments to characterize the interactions between the QscR receptor and a collection of compounds that alters QscR activity in cell-based reporter assays. We use the results of these investigations to propose models for a “signal transduction” mechanism between the QscR ligand binding domain and DNA binding domain upon ligand-induced agonism and antagonism. These models are tied to receptor levels and thus could have different implications for ligand behavior at non-quorate and quorate populations. Our findings also suggest limitations of cell-based reporter assays as proxies for biophysical parameters of QscR transcriptional activity, and we outline other experiments that may avoid these limitations. We anticipate the methods and models presented here should be broadly applicable to the LuxR-type receptor family.