Intercellular Communication in Marine Vibrio Species: Density-Dependent Regulation of the Expression of Bioluminescence

This chapter focuses on the different molecular mechanisms two model luminous bacteria, Vibrio fischeri (a symbiont) and V. harveyi (a free-living microbe), use for regulating lux expression. Expression of luminescence in most bacteria is tightly regulated by the density of the population. In V. fischeri, the regulatory genes involved in density-dependent control of luminescence are adjacent to the luxCDABEG operon encoding the luciferase enzymes. The regulatory genes that control luminescence in V. harveyi are different from those of V. fischeri. One complementation group of V. harveyi dim mutants could be restored to full light production by a family of recombinant cosmids containing a subset of common restriction fragments. Initial HAI-1 and HAI-2 signal recognition by LuxN and LuxQ could activate a series of phosphotransfer reactions. Two-component circuits have been characterized in which a single protein contains both a sensor kinase and a response regulator domain (similar to LuxN and LuxQ) and a second protein contains both a response regulator domain and a DNA binding motif (similar to LuxO). The differences between the regulatory circuits controlling density-dependent expression of luminescence in V. fischeri and V. harveyi are striking. Subsequent mutations and gene duplications and rearrangements generated new and multiple autoinducers, receptivities, and regulatory connections, finally resulting in a bacterium with the properties of V. harveyi.