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 la cZ reporter transcriptional fusion to the spvB structural gene on the single-copy virulence plasmid, it was found that while spvB transcript ion was induced in stationary-phase cultures, the induced level of exp ression was lower than that reported for the spv system in other serov ars of Salmonella. Surprisingly, inactivation of the gene encoding the positive activator SpvR resulted in only a threefold reduction in spv B transcription. In contrast, spvB transcription in stationary-phase c ultures was enhanced by 10-fold in mutants deficient in crp-encoded CR P or cya-encoded adenylate cyclase. Wild-type (i.e., 10-fold-lower) le vels of spvB expression were restored by providing active copies of np or cya on recombinant plasmids. Enhanced spvB transcription was not s een in crp or aa mutants in the absence of a functional spvR positive regulatory gene, showing that the cAMP-CRP system acted on spvB expres sion either in conjunction with or via SpvR. A lacZ transcriptional fu sion to spvR could not be induced in stationary-phase cultures in the absence of functional SpvR, regardless of the cAMP-CRP status of the c ells. 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 poorl y transcribed from the single-copy virulence plasmid in S. typhimurium LT2 and with a suppression of this defect via inactivation of the cAM P-CRP system. The physiological significance of cAMP-CRP involvement i n spv expression is discussed.