EVIDENCE THAT PRFA, THE PLEIOTROPIC ACTIVATOR OF VIRULENCE GENES IN LISTERIA-MONOCYTOGENES, CAN BE PRESENT BUT INACTIVE

All virulence genes of Listeria monocytogenes identified to date are p ositively regulated by PrfA, a transcriptional activator belonging to the Crp-Fnr family. Low temperature and cellobiose are two environment al signals known to repress expression of virulence genes in L. monocy togenes. In the present work, we analyzed the effect of temperature an d cellobiose on the expression of the PrfA protein. At low temperature , PrfA was undetected, although prfA monocistronic transcripts are pre sent. In contrast, PrfA was fully expressed in the presence of cellobi ose. These results strongly suggest that virulence gene activation dep ends on both the presence of PrfA and additional regulatory pathways t hat either modify PrfA or act synergistically with PrfA.