Negative regulation of PrfA, the key activator of Listeria monocytogenes virulence gene expression, is dispensable for bacterial pathogenesis.

Listeria monocytogenes is a facultative intracellular bacterial pathogen that regulates the expression of virulence-associated gene products in response to specific host cell compartment environments. The PrfA protein of L. monocytogenes functions as a key regulatory factor required for the differential expression of bacterial virulence gene products within infected host cells. PrfA both positively and negatively regulates its own expression, and while PrfA positive regulation is required for cell-to-cell spread of L. monocytogenes and for full virulence in infected mice, a role for negative regulation has been of presumed importance but has yet to be established. To address the role of negative regulation of prfA expression in L. monocytogenes pathogenesis, prfA promoter mutations designed to reduce or eliminate negative regulation were introduced into L. monocytogenes and analysed for their effects on patterns of PrfA-dependent gene expression and virulence in murine models of infection. High level PrfA production resulting from the prfA promoter mutations produced significantly increased levels of PrfA-regulated gene expression in broth-grown cultures; however the apparent loss of negative prfA regulation had no deleterious effects on growth and spread of the bacteria within infected tissue culture cells or on virulence in mice. The results indicate that while negative regulation of prfA expression exists and provides a feedback system for the control of PrfA synthesis, this feedback system is dispensable for virulence.