Identification of the central quorum sensing regulator of virulence in the enteric phytopathogen, Erwinia carotovora: the VirR repressor

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Abstract
In the Gram-negative phytopathogen, Erwinia carotovora ssp. atroseptica (Eca) virulence depends on the production of a N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) quorum sensing (QS) signal. This work identifies the elusive 'missing link' between QS and virulence in Erwinia. We have identified and characterized a novel regulator of virulence, VirR, in Eca and show that a virR mutation completely restores virulence factor production to an Eca mutant unable to synthesize OHHL. This effect of the virR mutation translates to a restoration of virulence to wild-type levels and thus provides evidence that VirR acts to prevent the production of virulence factors at low cell density. We also show that, in Eca, transcription of virulence genes is controlled by OHHL and that this control is effected through the action of VirR. We also demonstrate that the VirR regulatory pathway is present and functional in both blackleg and soft rotting species of Erwinia.