Quorum sensing ligand-receptor specificities in vibrios and photobacteria

Abstract

Quorum sensing is a process of bacterial cell-cell communication that relies on the production, release, and receptor-driven detection of extracellular signal molecules called autoinducers. Gram-negative bacteria often use small-molecule autoinducers and membrane-bound histidine sensor kinases for ligand detection and response. To understand signaling specificities in autoinducer synthase-receptor pairs, here, we characterize the Photobacterium angustum CqsA/CqsS system and the Vibrio harveyi LuxM/LuxN system.

Photobacterium genomes possess cqsA and cqsS genes, encoding the CAI-1 synthase and CqsS receptor, respectively. Many photobacterial cqsA genes harbor a conserved frameshift mutation. Correcting the P. angustum cqsA reading frame restores production of a mixture of CAI-1 molecules. P. angustum can overcome the cqsA frameshift to produce a variety of CAI-1 molecules under particular growth-limiting conditions presumably through a ribosome slippage mechanism. The P. angustum CqsS receptor detects CAI-1 autoinducers carrying C8 and C10 tails. The broad CAI-1 detection profile in P. angustum is determined by a Ser168 residue. We propose that P. angustum uses CAI-1 signaling for vibrio/photobacteria interspecies communication and adaptation to stressful environments. By contrast, V. harveyi exclusively produces and detects N-((R)-3-hydroxybutanoyl)-L-homoserine lactone (3OH-C4 HSL). To discover the principles underlying the exquisite selectivity LuxN receptor has for its ligand, we identified LuxN mutants with altered specificity. LuxN uses three mechanisms to verify that the bound molecule is the correct ligand: In the context of the overall ligand-binding site, His210 validates the C3 modification, Leu166 surveys the chain-length, and a strong steady-state kinase bias imposes an energetic hurdle for inappropriate ligands to elicit signal transduction. Mutations that bias LuxN to the agonized, Kinaseoff, state are clustered in a region adjacent to the ligand-binding site, suggesting that this region acts as the switch that triggers signal transduction. These LuxN properties likely maintain faithful intraspecies communication in V. harveyi. The analyses of these two systems illuminate how histidine sensor kinases differentiate between ligands and exploit those differences to regulate QS signaling activities.