The quorum-sensing regulation of Vibrio fischeri : novel components of the autoinducer/LuxR regulatory circuit

Abstract

In the marine bacterium Vibrio fischeri two intercellular homoserine-Iactone signal molecules (luxI-dependent 30C6-HSL and the ainS-dependent C8-HSL) and the transcriptional activator LuxR regulate the luminescence system in a cell-density dependent manner by a process termed quorum sensing. In this study, five additional proteins whose production is regulated by quorum sensing are described, and the genes encoding four of the five proteins, denoted as QsrP, RibB, QsrV, and AcfA, are analyzed. Each protein is positively regulated by 30C6-HSL and LuxR and negatively regulated at low population density by C8-HSL. Probable LuxR/autoinducer binding sites are found in the promoter region of each. QsrP and RibB are encoded monocistronically, whereas AcfA and QsrV appear to be encoded by a two-gene operon. On the basis of sequence similarity to proteins of known function from other organisms, RibB is believed to be an enzyme that catalyzes the transformation of ribulose 5-phosphate to 3,4-dihydroxy-2- butanone 4-phosphate, a precursor for the xylene ring of riboflavin; AcfA is believed to be a pilus subunit; and the functions of QsrP and QsrV are unknown at this time. A qsrP mutant was reduced in its ability to colonize its symbiotic partner, Euprymna scolopes when placed in competition with the parent strain. On the other hand, a mutant strain of V. fischeri containing an insertion in acfA, which is believed to be polar with respect to qsrV, displayed enhanced colonization competence in a competition assay. A ribB mutant grew well on media not supplemented with additional riboflavin and displayed normal induction of luminescence. Both phenotypes suggest that the lack of a functional ribB gene is complemented by another gene of similar function in the mutant. Oriented divergently from acfA are open reading frames that code for two putative proteins that are similar in sequence to members of the LysR family of transcriptional regulators. Organization of the two divergent sets of genes and the shared promoter region suggests that transcription of acfA and qsrV may be regulated by one or both of these divergently transcribed proteins. This work defines a quorum-sensing regulon in V. fischeri. A model describing its regulation is presented.