Direct Interaction between Quorum-sensing Regulator SmcR and RNA Polymerase Is Mediated by Integration Host Factor to Activate vvpE Encoding Elastase in Vibrio vulnificus

It has been suggested that quorum sensing is an important signal transduction system regulating the expression of numerous virulence genes in bacterial pathogens. We previously revealed that SmcR, a LuxR homologue of Vibrio vulnificus, activates promoter S, an RpoS-dependent promoter of vvpE encoding a potential virulence factor elastase and binds in vitro to a binding site centered at -196.5. In this study, chromatin immunoprecipitation assays and promoter deletion analyses demonstrated that SmcR binds to the vvpE regulatory region in vivo and directly interacts with RNAP for activation of the vvpE expression. A search for regulatory genes involved in the regulation of elastase production singled out ihfA, which encodes for a subunit of integration host factor (IHF). Levels of both elastase activity and vvpE transcript decreased significantly as a result of inactivation of ihfA, and primer extension analyses demonstrated that IHF regulates the vvpE transcription by activating PS. Direct binding of IHF to the two distinct binding sites centered at -174 and -131, respectively, was determined using an electrophoretic mobility shift assay and a DNase I protection assay. Chromatin immunoprecipitation assays revealed that the interaction of SmcR with RNAP in vivo was mediated by IHF. Collectively, the results proposed a model whereby IHF positions SmcR to contact RNAP by looping the vvpE regulatory DNA, thus allowing precise control of the expression level of VvpE during the pathogenesis of V. vulnificus.