Requirement for the molecular adapter function of StpA at the Escherichia coli bgl promoter depends upon the level of truncated H-NS protein

Truncated derivatives of the Escherichia coli nucleoid-associated protein H-NS that lack the DNA-binding domain remain competent for silencing of the cryptic bgI operon in vivo. Previous studies have provided evidence for the involvement of either the homologous nucleoid protein StpA or the alternative sigma factor RpoS in this unusual silencing mechanism. Here, we rationalize this apparent discrepancy. We show that two hns alleles (hns-205::Tn10 and hns60), which produce virtually identical aminoterminal fragments of H-NS, have very different requirements for StpA to mediate bgI silencing. The hns60 allele produces a high level of truncated H-NS, which can overcome the absence of StpA, whereas the lower level expressed by hns-205::Tn 10 requires StpA for silencing. Reversing the relative levels of the two H-NS fragments reverses their requirement for StpA to silence bgl transcription. This suggests that the amino-terminal fragment of H-NS can be targeted to DNA to mediate silencing by multiple protein-protein interactions. The high-specificity interaction with StpA can function at low levels of truncated H-NS, whereas an alternative mechanism, perhaps involving lower specificity interactions with another protein(s), is only functional when truncated H-NS is abundant. These findings have important implications for the involvement of other proteins in H-NS-dependent transcriptional repression.