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We have investigated the role of H-NS, one of the major components of the bacterial nucleoid, in the expression of the virF gene present on the large virulence plasmid of Shigella and enteroinvasive Escherichia coli in response to different environmental conditions. VirF is an AraC-like protein which activates at least two promoters, virB and virG, both repressed by H-NS. Band shift experiments reveal that the affinity of H-NS for the virF and virB promoters is comparable, while the affinity for the virG promoter is higher. Polyarylamide gel electrophoresis of three DNA fragments containing the virF, the virB and the virG promoters demonstrates, in agreement with computer predictions, that they have an intrinsically curved structure, confirming the preference of H-NS for bent DNA. In vivo transcriptional analysis of virF mRNA shows that H-NS negatively controls the expression of virF at 30°C. The expression of a virF-lacZ …