Mutational Analysis of the Quorum-Sensing Receptor LasR Reveals Interactions that Govern Activation and Inhibition by Nonlactone Ligands

Gram-negative bacteria use N-acyl L-homoserine lactone (AHL) quorum-sensing (QS) signals to regulate the expression of myriad phenotypes. Nonnative AHL analogs can strongly attenuate QS receptor activity and thereby QS signaling; however, we currently lack a molecular understanding of the mechanisms by which most of these compounds elicit their agonistic or antagonistic profiles. In this study, we investigated the origins of striking activity profile switches (i.e., receptor activator to inhibitor, and vice versa) observed upon alteration of the lactone head group in certain AHL analogs. Reporter gene assays of mutant versions of the Pseudomonas aeruginosa QS receptor LasR revealed that interactions between the ligands and Trp60, Tyr56, and Ser129 govern whether these ligands behave as LasR activators or inhibitors. Using this knowledge, we propose a model for the modulation of LasR by AHL analogs-encompassing a subtly different interaction with the binding pocket to a global change in LasR conformation.