Site of transcriptional activation of virB on the large plasmid of Shigella flexneri 2a by VirF, a member of the AraC family of transcriptional activators.

VirB plays a central role in the regulation of virulence of Shigella flexneri. It acts as a transcriptional activator and is itself transcriptionally activated by another virulence protein, VirF. Experiments were performed in order to identify the site upstream of virB at which VirF binds in order to activate transcription. Progressive 5' deletions of the DNA upstream of the transcription start point of virB were constructed by subcloning and Bal31 deletion. These deletion derivatives were cloned into the chloramphenicol acetyltransferase (CAT) reporter gene plasmid pKK232-8 and the resulting plasmids were analysed using a CAT activity assay. This allowed identification of minimal regions required for VirB promoter activity and regions required for full enhancement of promoter activity by VirF. A region approximately 100 bp upstream from the transcription start point of virB was identified as being necessary for full activation of this promoter by VirF. This region encompasses at least one inverted repeat which may play a role in transcription repression in the absence of the activator protein, VirF.