Shigella flexneri 2a strain 2457T expresses three members of the H-NS-like protein family: characterization of the Sfh protein
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Shigella flexneri 2a is known to express the H-NS nucleoid-structuring protein and the paralogous protein StpA. Using bioinformatic analysis we have now discovered a third member of the H-NS protein family, Sfh (S higella f lexneri H-NS-like protein), in strain 2457T. This protein is encoded by the sfh gene, which is located on a high-molecular-mass plasmid that is closely related to the self-transmissible plasmid R27. When expressed in Escherichia coli, the Sfh protein can complement an hns null mutation, restoring wild-type Bgl, porin protein, and mucoidy phenotypes, and wild-type expression of the fliC and proU genes. While a knockout mutation in the sfh gene alone had no effect on the expression of virulence genes in S. flexneri, an additive effect on virulence gene derepression was seen when the sfh lesion was combined with a mutation in hns. Over-expression of the sfh gene repressed expression of the VirB virulence regulatory protein and transcription of a VirB-dependent structural gene promoter. The purified Sfh protein bound specifically to DNA sequences containing the promoters of the virF and virB virulence regulatory genes. These findings show that Sfh has the ability to influence genetic events beyond the genetic element that encodes it, including the expression of the S. flexneri virulence genes. They raise the possibility of a triangular relationship among three closely related proteins with broad consequences for genetic events in the bacterium that harbours them.
KeywordsSfh protein Plasmid R27 H-NS protein StpA protein Shigella flexneri.
We thank Philippe Bertin for the E. coli strains BE1414 and YK3421, Maria Mavris for S. flexneri strains, and Pablo Garcia Bravo and Antonio Cerqueira for help with protein purification. This work was supported by European Training and Mobility of Researchers Award ERBFMRXCT98-0164, Wellcome Trust Award 064284/Z01/Z and Enterprise Ireland Award SC/2002/22. P. Deighan was supported by a Wellcome Trust Prize Fellowship. The research described in this paper was performed in compliance with current laws governing genetic experimentation in the Republic of Ireland.
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