The positive regulator CfaD overcomes the repression mediated by histone-like protein H-NS (H1) in the CFA/I fimbrial operon of Escherichia coli.

CFA/I fimbriae of enterotoxigenic Escherichia coli are expressed at 37 degrees C and not at 20 degrees C. Expression of CFA/I fimbriae requires two DNA regions (regions 1 and 2) which are separated by 40 kb on the wild type plasmid. Region 2 encodes a protein (CfaD) which activates the promoter in region 1. We investigated whether the histone-like protein H-NS (H1) of E.coli is involved in the temperature regulated expression of CFA/I fimbriae. As demonstrated recently with other temperature regulated genes, a mutation in the gene coding for this nucleoid-associated H-NS (H1) protein resulted in derepression of CFA/I expression. CFA/I fimbriae were now expressed both at 20 degrees C and 37 degrees C. More strinkingly, the positive regulator CfaD was not needed for CFA/I expression in an H-NS- strain. This indicates that CfaD diminishes an inhibitory effect of the H-NS nucleoid-associated protein. We also showed that in the H-NS- strain the CfaD protein still has a positive effect on the transcription of CFA/I.