The virulence of Salmonella typhimurium for mice is dependent on a plasmid-borne gene cluster termed spv. We previously determined that both S. typhimurium and Escherichia coli bacteria grown in a rich medium preferentially express the spv genes during the stationary phase of growth. In this study we evaluated the role of KatF, a putative sigma factor for starvation- and stationary phase-induced genes, in the expression of the spvB gene. The transcription of spvB in E. coli was compared in katF and wild-type backgrounds, using cloned spvB-lacZ and spvB-cat fusions. Expression of spvB was found to be greatly affected in katF mutants. Complementation experiments performed with the cloned katF gene confirmed that KatF is required for the expression of the S. typhimurium virulence gene spvB in E. coli.