Natural selection on the luxA gene of bioluminescent bacteria

Despite a growing literature of Vibrio, Photobacterium, Shewanella, and Photorhabdus biology, little is known of the function bioluminescence provides to these light-emitting bacteria. Proposed benefits of bioluminescence include evasion of predators or attraction of prey for symbiotic bacterial hosts through a distraction, a method of oxygen consumption to suffocate a host or reduce competition from obligate aerobes, a mechanism that stimulates DNA repair, or as a redox sink. We tested for the presence or absence of destabilizing selection on 31 physicochemical properties of the luxA gene of bacterial luciferase in relation to a phylogenetic hypothesis and the location of selection within the protein structure, in an attempt to further understand the evolution of bacterial bioluminescence and its importance to symbiosis. We show that amino acid properties most influenced by destabilizing selection include power to be at the C-terminal, chromatographic index, and isoelectric point. The location of destabilizing selection for isoelectric point within a phylogenetic context indicates that bacterial ecology has had an effect on the evolutionary history of luxA, while the presence of destabilizing selection for chromatographic index supports previous findings that bioluminescence in these species is sensitive to environmental osmolarity.