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 cholerae CqsA/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 angustum CqsA/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. angustum cqsA 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. angustum CqsS 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.