Contribution of Rapid Evolution of theluxR-luxIIntergenic Region to the Diverse Bioluminescence Outputs ofVibrio fischeriStrains Isolated from Different Environments

Abstract

ABSTRACTVibrio fischeriserves as a valuable model of bacterial bioluminescence, its regulation, and its functional significance. Light output varies more than 10,000-fold in wild-type isolates from different environments, yet dim and bright strains have similar organization of the light-producingluxgenes, with the activator-encodingluxRdivergently transcribed fromluxICDABEG. By comparing the genomes of bright strain MJ11 and the dimmer ES114, we found that theluxregion has diverged more than most shared orthologs, including those flankinglux. Divergence was particularly high in the intergenic sequence betweenluxRandluxI. Analysis of the intergenicluxregion from 18V. fischeristrains revealed that, with one exception, sequence divergence essentially mirrored strain phylogeny but with relatively high substitution rates. The bases conserved among intergenicluxR-luxIsequences included binding sites for known regulators, such as LuxR and ArcA, and bases of unknown significance, including a striking palindromic repeat. By using this collection of diverseluxR-luxIregions, we found that expression of PluxI-lacZbut not PluxR-lacZtranscriptional reporters correlated with the luminescence output of the strains from which the promoters originated. We also found that exchange of a small stretch of theluxI-luxRintergenic region between two strains largely reversed their relative brightness. Our results show that theluxR-luxIintergenic region contributes significantly to the variable luminescence output amongV. fischeristrains isolated from different environments, although other elements of strain backgrounds also contribute. Moreover, theluxsystem appears to have evolved relatively rapidly, suggesting unknown environment-specific selective pressures.