The two Listeria monocytogenes strains EGD and NCTC 7973 display a different regulation pattern of their PrfA-dependent genes. All PrfA-dependent genes from L. monocytogenes NCTC 7973 are much more efficiently transcribed in brain-heart infusion medium than those from strain EGD. Transcription of these genes in EGD is, however, induced after shift into Minimal Essential Medium (MEM) to a level that is comparable to that of strain NCTC 7973. Expression of the internalin gene (inlA) is also influenced by PrfA, but only one (P2) out of three mapped promoters is strictly dependent on PrfA. In contrast to the other PrfA-regulated genes, transcription of inlA even from the P2 promoter is reduced in both strains after shift to MEM. The prfA deletion mutant SLCC 53 complemented with multiple copies of prfA synthesizes large amounts of monocistronic prfA transcript, but there is no concomitant increase in the transcripts of the PrfA-dependent genes. However, upon a shift into MEM, transcription of the PrfA-dependent genes (with the exception of the inlA gene) is highly induced even in the absence of de novo protein synthesis. The PrfA proteins of the two studied L. monocytogenes strains differ in their ability to activate PrfA-dependent genes. This difference is probably the result of amino acid exchange(s) in the C-terminal part of these proteins. Strain EGD supplemented with multiple copies of prfA-7973 shows a similar regulation of the PrfA-dependent genes as strain NCTC 7973, whereas multiple copies of prfA-EGD introduced into strain EGD hardly change the rate of transcription of the PrfA-dependent genes. Our data thus suggest that PrfA-mediated gene expression depends not only on the amount of the PrfA protein and the 'quality' of the putative PrfA-binding sites, but also on the 'quality' of the PrfA protein which seems to be influenced by its amino acid composition and by environmental parameters.