CqsA-CqsS quorum-sensing signal-receptor specificity in Photobacterium angustum

Quorum sensing (QS) is a process of bacterial cell-cell communication that relies on the production, detection and population-wide response to extracellular signal molecules called autoinducers. The QS system commonly found in vibrios and photobacteria consists of the CqsA synthase/CqsS receptor pair. Vibrio choleraeCqsA/S synthesizes and detects (S)-3-hydroxytridecan-4-one (C10-CAI-1), whereas Vibrio harveyi produces and detects a distinct but similar molecule, (Z)-3-aminoundec-2-en-4-one (Ea-C8-CAI-1). To understand the signalling properties of the larger family of CqsA-CqsS pairs, here, we characterize the Photobacterium angustumCqsA/S system. Many photobacterial cqsA genes harbour a conserved frameshift mutation that abolishes CAI-1 production. By contrast, their cqsS genes are intact. Correcting the P.angustumcqsA reading frame restores production of a mixture of CAI-1 moieties, including C8-CAI-1, C10-CAI-1, Ea-C8-CAI-1 and Ea-C10-CAI-1. This signal production profile matches the P.angustumCqsS receptor ligand-detection capability. The receptor exhibits a preference for molecules with 10-carbon tails, and the CqsS Ser¹⁶⁸ residue governs this preference. P.angustum can overcome the cqsA frameshift to produce CAI-1 under particular limiting growth conditions presumably through a ribosome slippage mechanism. Thus, we propose that P.angustum uses CAI-1 signalling for adaptation to stressful environments.