Differential activation of virulence gene expression by PrfA, the Listeria monocytogenes virulence regulator

PrfA is a pleiotropic activator of virulence gene expression in the pathogenic bacterium Listeria monocytogenes. Several lines of evidence have suggested that a hierarchy of virulence gene activation by PrfA exists. This hypothesis was investigated by assessing the ability of PrfA to activate the expression of virulence gene fusions to lacZ in Bacillus subtilis. Expression of PrfA in this heterologous host was sufficient for activation of transcription at the hly, plcA, mpl, and actA promoters. Activation was most efficient at the divergently transcribed hly and plcA promoters. The putative PrfA binding site shared by these promoters is perfectly symmetrical and appears to represent the optimum sequence for target gene activation by PrfA. The activation of actA and mpl expression was considerably weaker and occurred more slowly than that observed at the hly and plcA promoters, suggesting that greater quantities of PrfA are required for productive interaction at these promoters. Interestingly, expression of an inlA-lacZ transcriptional fusion was very poorly activated by PrfA in B. subtilis, suggesting that other Listeria factors, in addition to PrfA, are required for PrfA-mediated activation at this promoter. Further support for the involvement of such factors was obtained by constructing and analyzing a prfA deletion mutant of L. monocytogenes. We observed that, in contrast to that of the other genes of the PrfA regulon, expression of inlA is only partially dependent on PrfA.