Molecular dissection of VirB, a key regulator of the virulence cascade of Shigella flexneri

The VirB protein is a key regulator of virulence gene expression in the facultative enteroinvasive pathogen Shigella flexneri. While genetic evidence has shown that it is required for activation of transcription of virulence genes located on a 230-kb plasmid in this bacterium, hitherto, evidence that VirB is a DNA-binding protein has been lacking. Although VirB shows extensive homology to proteins involved in plasmid partitioning, it does not resemble any known conventional transcription factor. Here we show for the first time that VirB binds to the promoter regions of the virulence genes in vivo. We also show that VirB forms dimeric and higher oligomeric structures both in vivo and in vitro and that this property is independent of DNA binding. The oligomerization activity of VirB is distributed over two, domains: a leucine zipper-like motif and a carboxyl-terminal domain likely to form triple coiled structures. VirB possesses a helix-turn-helix motif, which is required for DNA binding. The amino-terminal domain of the protein is also required for DNA binding and virulence gene activation. The possibility that VirB requires a co-factor for specific interaction with target promoters in vivo is discussed.