Regulation of Bioluminescence in Photobacterium leiognathi Strain KNH6

Abstract

Bacterial bioluminescence is taxonomically restricted to certain proteobacteria, many of which belong to the Vibrionaceae. In the most well-studied cases, pheromone signaling plays a key role in regulation of light production. However, previous reports have indicated that certain Photobacterium strains do not use this regulatory method for controlling luminescence. In this study, we combined genome sequencing with genetic approaches to characterize the regulation of luminescence in Photobacterium leiognathi strain KNH6, an extremely bright isolate. Using transposon mutagenesis and screening for decreased luminescence, we identified insertions in genes encoding components necessary for the luciferase reaction (lux, lum, and rib operons) as well as in nine other loci. These additional loci encode gene products predicted to be involved in the tricarboxylic acid (TCA) cycle, DNA and RNA metabolism, transcriptional regulation, and the synthesis of cytochrome c, peptidoglycan, and fatty acids. The mutagenesis screen did not identify any mutants with disruptions of predicted pheromone-related loci. Using targeted gene insertional disruptions, we demonstrate that under the growth conditions tested, luminescence levels do not appear to be controlled through canonical pheromone signaling systems in this strain. IMPORTANCEDespite the long-standing interest in luminous bacteria, outside a few model organisms, little is known about the regulation and function of luminescence. Light-producing marine bacteria are widely distributed and have diverse lifestyles, suggesting that the control and significance of luminescence may be similarly diverse. In this study, we apply genetic tools to the study of regulation of light production in the extremely bright isolate Photobacterium leiognathi KNH6. Our results suggest an unusual lack of canonical pheromone-mediated control of luminescence and contribute to a better understanding of alternative strategies for regulation of a key bacterial behavior. These experiments lay the groundwork for further study of the regulation and role of bioluminescence in P. leiognathi. Bacterial bioluminescence has been the subject of scientific study for over 300 years (1), and light-producing bacteria can be placed phylogenetically into the Vibrionaceae, Shewanellaceae, and Enterobacteriaceae families (reviewed in reference 2). Despite this long-standing interest in luminous bacteria, only in the past few decades has the genetic basis of light production been revealed. These relatively recent studies have focused mainly on a few model organisms, particularly Vibrio harveyi (Vibrio campbellii) and Vibrio fischeri (Aliivibrio fischeri), and have contributed immensely to our understanding of bacterial pheromone signaling or quorum sensing (3-5). However, it is known that in marine habitats, luminous bacteria are widely distributed (1) and can be found free living as well as host associated (6), and it has been observed that certain isolates of Photobacterium leiognathi do not appear to display cell...

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