Role for the histone-like protein H-NS in growth phase-dependent and osmotic regulation of sigma S and many sigma S-dependent genes in Escherichia coli.

The sigma S subunit of RNA polymerase (encoded by the rpoS gene) is the master regulator in a complex regulatory network that controls stationary-phase induction and osmotic regulation of many genes in Escherichia coli. Here we demonstrate that the histone-like protein H-NS is also a component of this network, in which it functions as a global inhibitor of gene expression during the exponential phase of growth. On two-dimensional gels, at least 22 sigma S-controlled proteins show increased expression in an hns mutant. H-NS also inhibits the expression of sigma S itself by a mechanism that acts at the posttranscriptional level. Our results indicate that relief of repression by H-NS plays a role in stationary-phase induction as well as in hyperosmotic induction of rpoS translation. Whereas certain sigma S-dependent genes (e.g., osmY) are only indirectly regulated by H-NS via its role in the control of sigma S expression, others are also H-NS-regulated in a sigma S-independent manner. (For this latter class of genes, rpoS hns double mutants show higher levels of expression than mutants deficient in rpoS alone.) In addition, we demonstrate that the slow-growth phenotype of hns mutants is suppressed in hns rpoS double mutants and that many second-site suppressor mutants that spontaneously arise from hns strains carry lesions that affect the expression of sigma S.