REGULATION OF HLY EXPRESSION IN LISTERIA-MONOCYTOGENES BY CARBON-SOURCES AND PH OCCURS THROUGH SEPARATE MECHANISMS MEDIATED BY PRFA

Expression of the PrfA-controlled virulence gene hly (encoding the por e-forming cytolysin listeriolysin) is under negative regulation by rea dily metabolized carbon sources in Listeria monocytogenes, However, th e hyperhemolytic strain NCTC 7973 exhibits deregulated hly expression in the presence of repressing sugars, raising the possibility that a d efect in carbon source regulation is responsible for its anomalous beh avior. We show here that the activity of a second glucose-repressed en zyme, cy-glucosidase, is 10-fold higher in NCTC 7973 than in 10403S. U sing hly-gus fusions, we show that the prfA allele from NCTC 7973 caus es deregulated hly-gus expression in the presence of sugars in either the wild-type or the NCTC 7973 background, while the 10403S prfA allel e restores carbon source regulation. However, the prfA genotype does n ot affect the regulation of alpha-glucosidase activity by repressing s ugars. Of the two mutational differences in PrfA, only a Gly145Ser cha nge is important for regulation of hly-gus, Therefore, NCTC 7973 and 1 0403S have genetic differences in at least two loci: one in prfA that affects carbon source regulation of virulence genes and another in an unidentified gene(s) that up-regulates alpha-glucosidase activity. We also show that the decrease in pH associated with utilization of sugar s negatively regulates hly-gus expression, although sugars can affect hly-gus expression by another mechanism that is independent of pH.