LuxR- and acyl-homoserine-lactone-controlled non-lux genes define a quorum-sensing regulon in Vibrio fischeri

The luminescence (lux) operon (luxICDABEG) of the symbiotic bacterium Vibri o fischeri is regulated by the transcriptional activator LuxR and two acyl- homoserine lactone (acyl-HSL) autoinducers (the luxI-dependent 3-oxo-hexano yl-HSL [3-oxo-C6-HSL] and the ainS-dependent octanoyl-HSL [C8-HSL]) in a po pulation density-responsive manner called quorum sensing. To identify quoru m-sensing-regulated (QSR) proteins different from those encoded by lux gene s, we examined the protein patterns of V. fischeri quorum-sensing mutants d efective in luxI, ainS, and luxR by two-dimensional polyacrylamide gel elec trophoresis, Five non-lux QSR proteins, QsrP, RibB, AcfA, QsrV, and QSR 7, were identified; their production occurred preferentially at high populatio n density, required both LuxR and 3-oxo-C6-HSL, and was inhibited by C8-HSL at low population density. The genes encoding two of the QSR proteins were characterized: qsrP directs cells to synthesize an apparently novel peripl asmic protein, and ribB is a homolog of the Escherichia coli gene for 3,4-d ihydroxy-2-butanone 4-phosphate synthase, a key enzyme for riboflavin synth esis. The qsrP and ribB promoter regions each contained a sequence similar to the lux operon lux box, a 20-bp region of dyad symmetry necessary for Lu xR/3-oxo-C6-HSL-dependent activation of lux: operon transcription. V. fisch eri qsrP and ribB mutants exhibited no distinct phenotype in culture. Howev er, a qsrP mutant, in competition with its parent strain, was less successf ul in colonizing Euprymna scolopes, the symbiotic host of V. fischeri. The newly identified QSR genes, together with the fur operon, define a LuxR/acy l-HSL-responsive quorum-sensing regulon in V. fischeri.