The virH operon is part of the broader virulence (vir) regulon that is essential for a bacterium's ability to transfer T-DNA to plant cells, causing crown gall disease. The virulence genes are typically located on the Ti (tumor-inducing) plasmid, and their expression is regulated in response to plant-derived signals. The bicistronic virH operon is composed of 2 genes - virH1 and virH2 whose proteins resemble those of the cytochrome P450-type mono-oxygenases family. Expression of this operon is induced by phenolic compounds e.g., acetosyringone (AS) and repressed by indole acetic acid (IAA) which acts as a competitive inhibitor of AS. This expression is controlled by virA which encodes for a transmembrane histidine protein kinase and virG which encodes a cytoplasmic response regulator. AS binds to transmembrane histidine protein kinase virA, resulting in phosphorylation of virG which then binds to the vir box (10 to 12bp sequences) in the promoter region and initiates transcription. VirH2 is reponsible for a O-demethylation reaction which converts ferulic acid (a plant phenolic defence compound) to caffeic acid, which is less toxic, thus facilitating pathogenicity. VirH1, sharing 20% identitiy to virH2, is also involved in metabolism of phenolic compounds, though different from those metabolized by virH2.

Virulence Pathway (virH operon activation) References