The sigma S-regulated gene spvR of Salmonella typhimurium encodes an autoregulatory protein required for transcriptional activation of the virulence operon spvABCD. A mutation in the histone-like protein H-NS, which negatively controls the sigma S level, has been reported to increase spv gene expression in S. typhimurium strain LT2. In agreement with this, we found that transcription of spvR and spvABCD was derepressed in hns strains of Escherichia coli and S. typhimurium. Moreover, levels of spv gene expression in hns rpoS double mutants were higher than expression levels in mutants deficient in rpoS alone, and were close to those measured in wild-type strains. This demonstrates that H-NS contributes to spv gene regulation independently of its function in controlling the sigma S level. Since the same start site was used for spvR gene transcription in wild-type as in hns and hns rpoS mutant strains, it is likely that the spvR promoter. spvRp1, can be recognized efficiently by an RNA polymerase containing sigma 70. The spvR promoter region shows an intrinsic DNA curvature that might be a determinant in H-NS- and/or sigma S-mediated control. A single amino acid substitution, Leu to Pro at position 265, abolished the regulatory function of SpvR in E. coli and Salmonella, implicating the C-terminal domain of SpvR in its structure and/or regulatory function. The spvR265 allele is not transcribed at detectable levels in hns or hns rpoS strains, suggesting that activation of spvRp1 in these strains remains dependent on SpvR. Thus, we propose a model for spvR gene regulation in which SpvR acts as a co-regulator of an RNA polymerase containing either sigma 70 (in the absence of H-NS) or sigma S, to induce transcriptional initiation at spvRp1. Moreover, growth-phase regulation of spv gene expression was maintained in hns and hns rpoS strains, indicating that an additional element, besides sigma S, is involved in the growth-phase regulation in rich medium.