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.
