Sequence and function of LuxU: a two-component phosphorelay protein that regulates quorum sensing in Vibrio harveyi

Vibrio harveyi regulates the expression of bioluminescence (lux) in respons e to cell density, a phenomenon known as quorum sensing. In V. harveyi, two independent quorum-sensing systems exist, and each produces, detects, and responds to a specific cell density-dependent autoinducer signal. The autoi nducers are recognized by two-component hybrid sensor kinases called LuxN a nd LuxQ, and sensory information from both systems is transduced by a phosp horelay mechanism to the response regulator protein LuxO, Genetic evidence suggests that LuxO-phosphate negatively regulates the expression of lumines cence at low cell density in the absence of autoinducers. At high cell dens ity, interaction of the sensors with their cognate autoinducers results in dephosphorylation and inactivation of the LuxO repressor. In the present re port, we show that LuxN and LuxQ channel sensory information to LuxO via a newly identified phosphorelay protein that we have named LuxU, LuxU shows s equence similarity to other described phosphorelay proteins, including BvgS , ArcB, and Ypd1. A critical His residue (His 58) of LuxU is required for p hosphorelay function.