LysR-Type Transcriptional Regulator VirR Responds to Temperature and pH and Directly Activates the Transcription of virS-Containing Operon in Rhodococcus equi

Rhodococcus equi-a facultative intracellular pathogen of macrophages-causes bronchopneumonia in foals and patients who are immunocompromised. Virulent strains of R. equi possess a virulence-associated plasmid, which encodes a 15- to 17-kDa surface protein called virulence-associated protein A (VapA). VapA expression is regulated by temperature and pH. Two transcriptional regulators, VirR and VirS, are involved in the transcriptional regulation of vapA. VirR regulates VapA expression through VirS. However, whether VirR directly regulates virS transcription is unclear. In this study, we examined VirR binding to the promoter region of the icgA operon, which contains virS, using the electrophoretic mobility shift assay and DNase I footprinting. VirR bound DNA fragments containing the virR-icgA intergenic region. Transcription from the promoter in this region was VirR-dependent and regulated by temperature and pH. The VirR-binding site contained the LysR-type transcriptional regulator-binding consensus motif, T-N₁₁-A. A point mutation (L98E) in the putative ligand-binding pocket of VirR constitutively activated the icgA promoter. However, no apparent difference was observed in the electrophoretic mobility shift assay and DNase I footprinting using the icgA promoter when L98E VirR was compared with wild-type VirR. A bacterial two-hybrid system identified an interaction between VirR and RpoA. Our data reveal that VirR binds the promoter of the icgA operon and directly activates its transcription. Furthermore, the regulation of VapA expression in response to temperature and pH is mediated by VirR.