Stoutland, I. M. | bioRxiv Science

bioRxiv2026

MrtR of Mesorhizobium tianshanense reveals both activation and inhibition mechanisms of a LuxR-type quorum sensing receptor

Quorum sensing (QS) enables common gram-negative bacteria to coordinate collective behaviors through small molecule signals, yet how these signals tune receptor activity remains incompletely understood. Here, we define a mechanism by which ligand structure controls function in a LuxR-type QS receptor. Using structural and biochemical analyses, we investigate MrtR from Mesorhizobium tianshanense and show that ligand acyl-chain length governs receptor assembly and activity. We present full-length structures of MrtR bound to activating and inhibitory ligands, revealing a switch in oligomeric state. Long-chain (C14) N-acyl L-homoserine lactones (AHLs) act as agonists by promoting intra- and inter-subunit interactions that lead to homodimerization and DNA binding. In contrast, shorter (C8) AHLs fail to promote these contacts, favoring a monomeric, inactive state. Ligands of intermediate length produce graded responses consistent with partial dimer stabilization. Biochemical measurements of DNA binding, thermostability, and oligomerization, together with targeted mutagenesis, support this model and establish the functional importance of key structural contacts. These findings provide the first structural comparison of a full-length LuxR-type receptor bound to both agonist and antagonist. Our findings expand the known structural and mechanistic diversity of the LuxR family and suggest mechanistic similarities between structurally distinct receptors. SIGNIFICANCEQuorum sensing (QS) regulates diverse bacterial behaviors, and LuxR-type receptors are attractive targets for applications ranging from antivirulence strategies to synthetic biology and agriculture. Despite intense interest in developing chemical modulators of these systems, the molecular basis by which small molecules agonize or antagonize LuxR-type receptors remains poorly understood. Here, we investigate the LuxR-type receptor MrtR and report crystal structures of the full-length receptor bound to an agonist and an antagonist, revealing how structurally similar compounds produce opposing outcomes. Notably, MrtR exhibits an unprecedented dimerization interface mediated by a ligand-responsive loop that undergoes large conformational changes. These findings establish a new structural framework for understanding signal discrimination in LuxR-type receptors and may enable rational reprogramming of QS in natural and engineered systems.

biochemistry↗

bioRxiv2026

Domain-swapped LuxR-type quorum sensing receptors reveal divergent ligand-response mechanisms among homologs

Many common bacteria use quorum sensing to regulate cell density-dependent phenotypes, including luminescence, biofilm formation, virulence, and symbiosis. The LuxI/R system is the best-characterized quorum sensing pathway in Gram-negative bacteria and consists of a LuxI-type synthase that produces an N-acyl L-homoserine lactone (AHL) autoinducer and a LuxR-type transcription factor that is regulated by AHL binding. Binding of native AHL signal promotes DNA binding and transcriptional regulation in some LuxR homologs (associative-type), while other homologs regulate transcription in the absence of ligand and are inactivated by native signal binding (dissociative-type). To better characterize what features determine ligand-response type, we generated structural mutants of two associative receptors (LasR of Pseudomonas aeruginosa and MrtR of Mesorhizobium tianshanense) and two dissociative receptors (EsaR of Pantoea stewartii and ExpR2 of Pectobacterium versatile). Swapping domains between these receptors revealed that the ligand-binding domain primarily determines associative vs. dissociative activity in response to native AHL agonists. Further, non-native AHL-derived antagonists maintained their activity profiles in receptors with interchanged DNA-binding domains. We also found that the extended linker between domains observed in the dissociative receptors does not determine mechanism of ligand response, and that inter-domain interactions may play an important role in activation for some receptors but not others. Notably, deletion of just one residue from the dissociative receptor EsaR produced a mutant with associative activity, the first time such mechanism switching has been reported for a LuxR-type receptor. These findings illuminate features essential for ligand response and highlight the mechanistic diversity of the LuxR family. IMPORTANCELuxI/R quorum sensing regulates various cell density-dependent phenotypes in Gram-negative bacteria. Prior research has developed small molecule modulators of LuxR-type receptors, with potential applications in anti-virulence, anti-biofouling, and bioengineering. Competitive antagonists have been reported for receptors active in the presence of native ligand but not for receptors active in its absence. A lack of knowledge about the molecular mechanisms of receptor response to ligand limits both our fundamental understanding of the LuxI/R quorum sensing process and the rational design of chemical modulators with superior activity profiles. We used a structural mutagenesis strategy with four LuxR-type receptors that operate via two distinct mechanisms to begin to dissect the structural features that drive differences in ligand response between receptors. These insights could aid in efforts to characterize novel LuxR homologs, understand potential interspecies communication via quorum sensing, and develop improved chemical probes to alter LuxR-type receptor activity.

microbiology↗