The Salmonella typhimurium virulence plasmid encodes a positive regulator of a plasmid-encoded virulence gene

The 90-kb virulence plasmid of Salmonella typhimurium is necessary for invasion beyond the Peyer's patches to the mesenteric lymph nodes and spleens of orally inoculated mice. Two Tn5 insertions located on the left side of a previously identified 14-kb virulence region (P. A. Gulig and R. Curtiss III, Infect. Immun. 58:3262-3271, 1988) and mapping 272 bp from each other exhibited opposite effects on splenic infection of mice after oral inoculation. spvR23::Tn5 decreased splenic infection by 1,000-fold, whereas a spv-14::Tn5 mutant outcompeted wild-type S. typhimurium for splenic infection by 27-fold in mice fed mixtures of mutated and wild-type S. typhimurium. spvR23::Tn5 was complemented by a virulence plasmid subclone with an insert sequence encoding only an 891-bp open reading frame specifying a 33,000-molecular-weight protein. The amino acid sequence of this open reading frame had significant homology to members of the LysR family of positive regulatory proteins; thus, the gene was named spvR (salmonella plasmid virulence). To examine the possible regulatory effects of spvR on other virulence genes, we constructed a lacZ operon fusion in a downstream virulence gene, spvB. When spvR subcloned behind the lac promoter was provided on a separate plasmid in trans to the spvB-lacZ operon fusion, transcription of spvB increased 15-fold. spv-14::Tn5, which conferred a competitive advantage to S. typhimurium, increased the expression of a spvR-lacZ operon fusion in cis. spvR is therefore a positive regulator of spvB and an essential virulence gene of S. typhimurium. As opposed to having spvR subcloned behind the lac promoter, the wild-type spvR gene present on the virulence plasmid did not function to positively regulate spvB-lacZ in trans when salmonellae were grown to the log phase in L broth, suggesting that this regulatory system is activated in vivo during infection.