The spv virulence operon of Salmonella typhimurium LT2 is regulated negatively by the cyclic AMP (cAMP)-cAMP receptor protein system.

The cyclic AMP (cAMP) receptor protein (CRP) was found to play a role in the growth phase regulation of the spv operon on the high-molecular-weight virulence plasmid of Salmonella typhimurium LT2. By using a lacZ reporter transcriptional fusion to the spvB structural gene on the single-copy virulence plasmid, it was found that while spvB transcription was induced in stationary-phase cultures, the induced level of expression was lower than that reported for the spv system in other serovars of Salmonella. Surprisingly, inactivation of the gene encoding the positive activator SpvR resulted in only a threefold reduction in spvB transcription. In contrast, spvB transcription in stationary-phase cultures was enhanced by 10-fold in mutants deficient in crp-encoded CRP or cya-encoded adenylate cyclase. Wild-type (i.e., 10-fold-lower) levels of spvB expression were restored by providing active copies of crp or cya on recombinant plasmids. Enhanced spvB transcription was not seen in crp or cya mutants in the absence of a functional spvR positive regulatory gene, showing that the cAMP-CRP system acted on spvB expression either in conjunction with or via SpvR. A lacZ transcriptional fusion to spvR could not be induced in stationary-phase cultures in the absence of functional SpvR, regardless of the cAMP-CRP status of the cells. When SpvR was provided in trans, transcription of the spvR-lacZ fusion was induced to similar levels in stationary-phase cultures with and without cAMP-CRP. These data are consistent with spvR being poorly transcribed from the single-copy virulence plasmid in S. typhimurium LT2 and with a suppression of this defect via inactivation of the cAMP-CRP system. The physiological significance of cAMP-CRP involvement in spv expression is discussed.