Alternate SlyA and H‐NS nucleoprotein complexes control hlyE expression in Escherichia coli K‐12

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Abstract
Summary: Haemolysin E is a cytolytic pore‐forming toxin found in several Escherichia coli and Salmonella enterica strains. Expression of hlyE is repressed by the global regulator H‐NS (histone‐like nucleoid structuring protein), but can be activated by the regulator SlyA. Expression of a chromosomal hlyE–lacZ fusion in an E. coli slyA mutant was reduced to 60% of the wild‐type level confirming a positive role for SlyA. DNase I footprint analysis revealed the presence of two separate SlyA binding sites, one located upstream, the other downstream of the hlyE transcriptional start site. These sites overlap AT‐rich H‐NS binding sites. Footprint and gel shift data showed that whereas H‐NS prevented binding of RNA polymerase (RNAP) at the hlyE promoter (PhlyE), SlyA allowed binding of RNAP, but inhibited binding of H‐NS. Accordingly, in vitro transcription analyses showed that addition of SlyA protein relieved H‐NS‐mediated repression of hlyE. Based on these observations a model for SlyA/H‐NS regulation of hlyE expression is proposed in which the relative concentrations of SlyA and H‐NS govern the nature of the nucleoprotein complexes formed at PhlyE. When H‐NS is dominant RNAP binding is inhibited and hlyE expression is silenced; when SlyA is dominant H‐NS binding is inhibited allowing RNAP access to the promoter facilitating hlyE transcription.