Regulation of quorum sensing in Vibrio harveyi by LuxO and Sigma-54

The bioluminescent marine bacterium Vibrio harveyi controls light productio n (lux) by an elaborate quorum-sensing circuit. V. harveyi produces and res ponds to two different autoinducer signals (AI-1 and AI-2) to modulate the luciferase structural operon (luxCDABEGH) in response to changes in cell-po pulation density. Unlike all other Gram-negative quorum-sensing organisms, V. harveyi regulates quorum sensing using a two-component phosphorylation-d ephospharylation cascade. Each autoinducer is recognized by a cognate hybri d sensor kinase (called LuxN and LuxQ). Both sensors transduce information to a shared phosphorelay protein called LuxU, which in turn conveys the sig nal to the response regulator protein LuxO. Phospho-LuxO is responsible for repression of luxCDABEGH expression at low cell density. In the present st udy, we demonstrate that LuxO functions as an activator protein via interac tion with the alternative sigma factor, sigma(54) (encoded by rpoN). Our re sults suggest that LuxO, together with sigma(54), activates the expression of a negative regulator of luminescence. We also show that phenotypes other than lux are regulated by LuxO and sigma(54), demonstrating that in Vibrio harveyi, quorum sensing controls multiple processes.