The spv region of Salmonella virulence plasmids is essential for the development of a systemic infection in mice. Transcriptional activation of the spvABCD operon occurs during stationary growth phase and is mediated by the regulatory gene product SpvR. We have previously shown that expression of a spvRAB'-cat fusion in Escherichia coli was dependent on the katF (rpoS) locus which encodes an alternative sigma factor (sigma S). The katF gene from Salmonella typhimurium has been cloned, sequenced, and used to construct Salmonella katF mutants by allelic replacement. Using these mutants, we demonstrated by mRNA and gene fusion analyses that sigma S, in conjunction with SpvR, controls the transcription of the regulatory gene spvR. In a second series of experiments, we sought to clarify the relationship between sigma S and SpvR in the control of spvABCD transcription. It was shown that expression of a transcriptional spvAB'-lacZ fusion could be restored in E. coli and Salmonella katF mutants when spvR was expressed in trans from an exogenous promoter. Moreover, identical spvA mRNA startpoints were detected in katF+ and katF strains. These results indicate that the reduction of spvABCD transcription in katF mutants is mainly due to decreased expression of spvR. Finally, mouse inoculation studies with S. typhimurium katF mutants of both wild-type and virulence plasmid-cured strains suggest that katF contributes to Salmonella virulence via the regulation of chromosomal genes in addition to that of spv genes
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