VirR, a response regulator critical for Listeria monocytogenes virulence

Signature-tagged mutagenesis (STM) was used to identify new genes involved in the virulence of the Gram-positive intracellular pathogen Listeria monocytogenes. One of the mutants isolated by this technique had the transposon inserted in virR, a gene encoding a putative response regulator of a two-component system. Deletion of virR severely decreased virulence in mice as well as invasion in cell-culture experiments. Using a transcriptomic approach, we identified 12 genes regulated by VirR, including the dlt-operon, previously reported to be important for L. monocytogenes virulence. However, a strain lacking dltA, was not as impaired in virulence as the D virR strain, suggesting a role in virulence for other members of the vir regulon. Another VirR-regulated gene is homologous to mprF, which encodes a protein that modifies membrane phosphatidyl glycerol with L-lysine and that is involved in resistance to human defensins in Staphylococcus aureus. VirR thus appears to control virulence by a global regulation of surface components modifications. These modifications may affect interactions with host cells, including components of the innate immune system. Surprisingly, although controlling the same set of genes as VirR, the putative cognate histidine kinase of VirR, VirS, encoded by a gene located three genes downstream of virR, was shown not to be essential for virulence. By monitoring the activity of VirR with a GFP reporter construct, we showed that VirR can be activated independently of VirS, for example through a mechanism involving variations in the level of intracellular acetyl phosphate. In silico analysis of the VirR-regulated promoters revealed a VirR DNA-binding consensus site and specific interaction between purified VirR protein and this consensus sequence was demonstrated by gel mobility shift assays. This study identifies a second key virulence regulon in L. monocytogenes, after the prfA regulon.