Shigella flexneri is a gram negative, rod shaped bacterium that is the causative agent of bacillary dysentery, which is characterized by fever, abdominal pain, and bloody diarrhea. Genes essential to the pathogenicity of S. flexneri are encoded by a virulence plasmid. Shigella has evolved a complex regulatory system to regulate transcription of virulence genes. This involves two regulators, VirF and VirB, which allow the bacterium to respond to environmental stimuli and maximally exploit host niches. An additional factor impacting virulence gene regulation is H-NS, a histone nucleoid structuring protein that globally represses transcription. This work addresses the transcriptional regulation of icsP and ospZ, divergent virulence plasmid encoded genes. Analysis of the intergenic region shared by icsP and ospZ has identified remote VirB binding sites essential to icsP promoter activity. Analyses of the requirements for VirB-dependent regulation of icsP promoter activity identified the VirB binding sites as a cis -acting element with small spacing requirements. In addition, analyses of H-NS-dependent regulation of the icsP promoter showed that intrinsic curvature does not have a role in H-NS mediated repression. Together, these data indicate that the VirB and H-NS interact to modulate icsP promoter activity, demonstrating that transcriptional regulation is an intricate process
