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Expression of the PrfA-controlled virulence genehly (encoding the pore-forming cytolysin listeriolysin) is under negative regulation by readily metabolized carbon sources inListeria monocytogenes . However, the hyperhemolytic strain NCTC 7973 exhibits deregulatedhly expression in the presence of repressing sugars, raising the possibility that a defect in carbon source regulation is responsible for its anomalous behavior. We show here that the activity of a second glucose-repressed enzyme, α-glucosidase, is 10-fold higher in NCTC 7973 than in 10403S. Usinghly-gus fusions, we show that theprfA allele from NCTC 7973 causes deregulatedhly-gus expression in the presence of sugars in either the wild-type or the NCTC 7973 background, while the 10403SprfA allele restores carbon source regulation. However, theprfA genotype does not affect the regulation of α-glucosidase activity by repressing sugars. Of the two mutational differences in PrfA, only a Gly145Ser change is important for regulation ofhly-gus . Therefore, NCTC 7973 and 10403S have genetic differences in at least two loci: one inprfA that affects carbon source regulation of virulence genes and another in an unidentified gene(s) that up-regulates α-glucosidase activity. We also show that the decrease in pH associated with utilization of sugars negatively regulateshly-gus expression, although sugars can affecthly-gus expression by another mechanism that is independent of pH.

Regulation of hly Expression in Listeria monocytogenes by Carbon Sources and pH Occurs through Separate Mechanisms Mediated by PrfA