RELATIONSHIPS BETWEEN H-NS, SIGMA(S), SPVR AND GROWTH-PHASE IN THE CONTROL OF SPVR, THE REGULATORY GENE OF THE SALMONELLA PLASMID VIRULENCEOPERON

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 in crease spy gene expression in S. typhimurium strain LT2. In agreement with this, we found that transcription of spvR and spvABCD was derepre ssed in hns strains of Escherichia coli and S. typhimurium . Moreover, levels of spy gene expression in hns rpoS double mutants were higher than expression levels in mutants deficient in rpoS alone, and were cl ose to those measured in wild-type strains. This demonstrates that H-N S contributes to spy 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 st rains, it is likely that the spvR promoter, spvRp1, can be recognized efficiently by an RNA polymerase containing a. The spvR promoter regio n 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 i n its structure and/or regulatory function. The spvR(265) allele is no t transcribed at detectable levels in hns or hns rpoS strains, suggest ing that activation of spvRp1 in these strains remains dependent on Sp vR. Thus, we propose a model for spvR gene regulation in which SpvR ac ts as a co-regulator of an RNA polymerase containing either sigma (70) (in the absence of H-NS) or sigma(S), to induce transcriptional initi ation at spvRp1. Moreover, growth-phase regulation of spy gene express ion was maintained in hns and hns rpoS strains, indicating that an add itional sigma(S) is involved in the growth-phase regulation in rich me dium.