DIFFERENTIAL ACTIVATION OF VIRULENCE GENE-EXPRESSION BY PRFA, THE LISTERIA-MONOCYTOGENES VIRULENCE REGULATOR

PrfA is a pleiotropic activator of virulence gene expression in the pa thogenic bacterium Listeria monocytogenes. Several lines of evidence h ave suggested that a hierarchy of virulence gene activation by PrfA ex ists, This hypothesis was investigated by assessing the ability of Prf A to activate the expression of virulence gene fusions to lacZ in Baci llus subtilis. Expression of PrfA in this heterologous host was suffic ient for activation of transcription at the hly, plcA, mpl, and actA p romoters. Activation was most efficient at the divergently transcribed hly and plcA promoters. The putative PrfA binding site shared by thes e promoters is perfectly symmetrical and appears to represent the opti mum sequence for target gene activation by PrfA, The activation of act A and mpl expression was considerably weaker and occurred more slowly than that observed at the hly and plcA promoters, suggesting that grea ter quantities of PrfA are required for productive interaction at thes e promoters. Interestingly, expression of an inlA-lacZ transcriptional fusion was very poorly activated by PrfA in B, subtilis, suggesting t hat other Listeria factors, in addition to PrfA, are required for PrfA -mediated activation at this promoter, Further support for the involve ment 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 on ly partially dependent on PrfA.