Introduction: During infection, Staphylococcus aureus is exposed to reactive electrophilic species, such as quinones and aldehydes. The MhqR regulon was previously shown to confer resistance towards methylhydroquinone (MHQ) and quinone-like antimicrobials, such as pyocyanin and ciprofloxacin [1]. However, the quinone-sensing mechanism of the MhqR repressor is unknown thus far. Question: We hypothesized that quinones bind to a ligand-binding pocket of MhqR, leading to its inactivation and derepression of transcription of quinone detoxification genes. Materials & Methods: To investigate the quinone-sensing mechanism of MhqR, we resolved the crystal structure of MhqR in the presence of MHQ and conducted ITC analyses, DNA binding assays and phenotype analyses. Results: The crystal structure of MhqR in complex with MHQ identified potential quinone interaction sites, which supported the predictions of the potential quinone binding pocket. Transcriptional analysis identified F11, R16, F39, and H111 of MhqR as essential for MHQ sensing in vivo, while L115 is important for DNA binding in vivo. In phenotype analyses of mhqR variants, the mhqR-F11A, mhqR-R16A, mhqR-F39A, and mhqR-H111A mutants were impaired in growth under MHQ stress in S. aureus. However, MHQ binding was not affected in the MhqR variants in vitro, as shown by ITC experiments and EMSAs in vitro. Summary: Structural and mutational analyses identified a quinone-binding pocket of MhqR, which is essential for MHQ sensing in vivo. Our results provide novel insights into the redox-mechanism of MhqR, that contributes to antimicrobial resistance in S. aureus.

Reference:

[1] Fritsch VN, Loi VV, Busche T, Sommer A, Tedin K, Nürnberg DJ, Kalinowski J, Bernhardt J, Fulde M, Antelmann H. The MarR-type repressor MhqR confers quinone and antimicrobial resistance in Staphylococcus aureus. Antioxid Redox Signal. doi: 10.1089/ars.2019.7750 (2019)