Growth phase and SpvR regulation of transcription of Salmonella typhimurium spvABC virulence genes.

The 90 kb virulence plasmid of Salmonella typhimurium is required for bacterial growth beyond the small intestine to deeper tissues such as the spleen and liver of orally inoculated mice. We constructed transcriptional lacZ fusions within the cloned plasmid-borne virulence genes spvA, spvB and spvC of S. typhimurium to demonstrate that spvR encodes a trans-acting positive regulator for the transcription of spvA, spvB and spvC. Data suggesting that the activation of spvABC transcription is dependent on the growth phase of both S. typhimurium and Escherichia coli grown in Luria Broth (LB) are also presented. Complementation experiments for virulence in mice confirmed that at least spvR and spvC are virulence genes and further suggested that the spvRABC gene cluster consists of at least three transcriptional units containing spvR, spvC and spvABC, respectively. Reinitiation of transcription at spvC was confirmed in vitro, using a lacZ fusion, and was shown to be independent of SpvR-mediated control in LB.