Characterization of translation termination mutations in the spv operon of the Salmonella virulence plasmid pSDL2.

The spv region of the Salmonella virulence plasmids consists of five genes located on an 8-kb fragment previously shown to be essential for virulence in mice. Four structural genes, spvABCD, form an operon that is transcriptionally activated by the spvR gene product in the stationary phase of growth. The role of the individual spv genes in the virulence phenotype was tested by isolating translation termination linker insertions in each gene. Analysis of proteins synthesized in minicells identified each of the spvABCD gene products and confirmed the dependence of spv structural gene expression on the SpvR regulatory protein. The oligonucleotide insertions in spvA, -B, and -C were shown to be nonpolar. Virulence testing indicated that the SpvB protein, regulated by SpvR, is essential for Salmonella dublin to cause lethal disease in mice. Inserts in spvC and spvD were unstable in vivo for unknown reasons, but these mutants still killed mice at slightly higher inocula. Abolition of spvA had no effect on virulence in this system.