Transcriptional regulation of Pseudomonas aeruginosa rhlR: role of the CRP orthologue Vfr (virulence factor regulator) and quorum-sensing regulators LasR and RhlR

Abstract

The production of many virulence factors byPseudomonas aeruginosais regulated by the quorum-sensing (QS) response. In this regulatory network LasR and RhlR, bound to their corresponding autoinducers, play a central role. The QS response has a hierarchical structure: LasR/3O-C12-HSL activates the transcription ofrhlR, and RhlR/C4-HSL activates the transcription of several genes, including therhlABoperon, which encodes the enzymes responsible for rhamnolipid synthesis. TherhlABoperon is located immediately upstream of therhlRgene.rhlRhas four transcription start sites, two of which are located in therhlBcoding region. Vfr directly activates transcription oflasR, and has been reported to be also involved inrhlRexpression. The aim of this work was to characterize the details of the mechanism ofrhlRtranscriptional regulation. We show that Vfr directly regulatesrhlRtranscription through its binding to several Vfr-binding sites (VBSs) present in therhlRpromoter region, one of which has a negative effect on transcription. Two of the VBSs overlap withlasboxes where LasR/3O-C12-HSL binds to activaterhlRtranscription. We also show thatrhlRtranscription is subject to positive-feedback autoregulation through RhlR/C4-HSL activation of therhlApromoter. This positive autoregulation plays a major role inrhlRexpression.