POSITIVE REGULATION OF SHIGELLA-FLEXNERI VIRULENCE GENES BY INTEGRATION HOST FACTOR

In Shigella flexneri, expression of the plasmid-encoded virulence gene s is regulated via a complex cascade involving DNA topology, specific transactivators, and the nucleoid-associated protein H-NS, which repre sses transcription under inappropriate environmental conditions. We ha ve investigated the involvement of a second nucleoid-associated protei n, integration host factor (IHF), in virulence gene expression. We fou nd that transcription of the invasion-specific genes is repressed in a strain harboring an ihfA mutation, particularly on entry into the sta tionary phase. Expression of the virB gene, whose product is required for the activation of these structural genes, is also enhanced by IHF in the stationary phase. In contrast, the virF gene, which encodes an activator of virB, is stimulated by IHF in both the logarithmic and ea rly stationary phases of growth, as is another virF-regulated gene, ic sA. We have identified regions of the virF, virB, and icsA promoters w hich form IHF-dependent protein-DNA complexes in vitro and have locate d sequences within these regions with similarity to the consensus IHF binding site. Moreover, results from experiments in which the virF or virB gene was expressed constitutively confirm that IHF has a direct i nput at the level of both virF and virB transcription. Finally, we pro vide evidence that at the latter promoter, the primary role of IHF may be to overcome repression by the H-NS protein, To our knowledge, this is the first report of a role for IHF in controlling gene expression in S. flexneri.