In the economically important phytopathogen, Pectobacterium atrosepticum, expression of plant cell wall degrading enzymes and other virulence determinants is controlled in a cell density-dependent fashion, termed quorum sensing (QS). Canonical QS systems in Gram-negative bacteria contain a LuxI-type protein, synthesizing a signalling molecule, and a LuxR-type regulator, responding to the signalling molecule above threshold concentrations. In P. atrosepticum, the central LuxR-type repressor of virulence, VirR, has been identified and its impacts on virulence characterized. Here we define the broader VirR regulon using chromatin immunoprecipitation (ChIP) and in planta microarrays. Ninety-four direct VirR targets were identified by ChIP microarrays and a consensus VirR binding site was determined. Purified VirR was used in DNA gel shift assays on target promoters and VirR : promoter binding was disrupted by exogenous addition of the signalling molecule, N-(3-oxohexanoyl)-l-homoserine lactone (OHHL). VirR autorepressed, and directly activated the transcription of rsmA in the absence of OHHL. Finally, we showed that VirR directly regulated the production of siderophores and controlled swimming motility. This is the first report characterizing the direct targets of VirR and provides clear evidence that this LuxR-type protein can act in vivo as both an activator and repressor of transcription in the absence of its cognate signalling molecule.
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