A truncated H-NS-like protein from enteropathogenic Escherichia coli acts as an H-NS antagonist

The H-NS nucleoid-associated protein of Escherichia coli is the founder member of a widespread family of gene regulatory proteins which have a bipartite structure, consisting of an N-terminal coiled-coil oligomerization domain and a C-terminal DNA-binding domain. Here we characterize a family of naturally occurring truncated H-NS derivatives lacking the DNA-binding domain, which we term the H-NST family. H-NST proteins are found in large genomic islands in pathogenic E. coli strains, which are absent from the corresponding positions in the E. coli K-12 genome. Detailed analysis of the H-NST proteins from enteropathogenic E. coli (EPEC) and uropathogenic E. coli (UPEC) shows that the EPEC protein (H-NST(EPEC)) has a potent anti-H-NS function at the classical H-NS-repressed operon proU. This correlates with the ability of H-NST(EPEC) to co-purify with H-NS in vitro, and can be abolished by a mutation of leucine 30 to proline which is predicted to prevent the N-terminal region from forming a coiled-coil structure. In contrast, despite being 90% identical to H-NST(EPEC) at the protein level, the UPEC homologue (H-NST(UPEC)) has only a weak anti-H-NS activity, correlating with a much-reduced ability to interact with H-NS during column chromatography. A single amino acid difference at residue 16 appears to account for these different properties. The hnsT(EPEC) gene is transcribed monocistronically and expressed throughout the exponential growth phase in DMEM medium. Our data suggest that a truncated derivative of H-NS encoded by an ancestral mobile DNA element can interact with the endogenous H-NS regulatory network of a bacterial pathogen.