sbmC, an Escherichia coli gene, belongs to the SOS regulon, whose product is involved in cell susceptibility to microcin B17 and its expression is induced at the onset of the stationary growth phase. In the present work, we have investigated the regulation of sbmC expression during SOS induction and the stationary growth phase using a single-copy sbmC'-'lacZ fusion. The SOS induction of sbmC is profoundly diminished in the hns mutant and less diminished in the rpoS mutant. The strain with hns, rpoS double mutation, showed a similar level of sbmC induction to that of a strain with hns single mutation. Mutation in rpoS or hns causes the repression of the sbmC gene during the stationary growth phase. The sbmC expression in the rpoS mutant strain was approximately twofold lower than that in the hns mutant and the rpoS hns double mutant showed a similar level of sbmC expression to mutants deficient in rpoS alone. Interestingly, the sbmC'-'lacZ expression in the exponential growth phase was not derepressed in the hns mutant background. Transformation of hns and rpoS mutants with plasmids carrying histone-like nucleoid protein (HNS) and RpoS effectively restored the sbmC expression to the wild-type level, respectively. The gel mobility shift assay showed that purified H-NS protein directly bound with a high affinity to a DNA fragment carrying the sbmC promoter region. These findings demonstrate that H-NS regulates the sbmC expression via H-NS's direct binding to the promoter region. In conclusion, our data suggest that H-NS and RpoS regulate a stationary phase-inducible sbmC gene in E. Coli.