The role of MerA in the defense against the thiol-specific oxidant hypothiocyanous acid (HOSCN) in Staphylococcus aureus

Introduction: Staphylococcus aureus colonizes the skin and the airways of the healthy human population, but can also lead to life-threatening infections. During colonization and phagocytosis by neutrophils, S. aureus encounters the oxidant HOSCN. Goals: Here, we aimed to characterize the defense mechanisms against HOSCN in S. aureus. Materials & Methods: RNA-seq analyses, redox biosensor measurements, mutant phenotypes and crystal structure analysis were used to investigate the HOSCN defense mechanisms. Results: In the transcriptome, HOSCN caused a strong thiol-specific oxidative, electrophile and metal stress responses as well as protein damage in S. aureus, as indicated by the upregulation of the HypR, TetR1, PerR, QsrR, MhqR, CstR, CsoR, CzrA, AgrA, HrcA and CtsR regulons. Brx-roGFP2 biosensor measurements and Western blot analyses revealed a reversible oxidative shift of the bacillithiol redox potential (EBSH) and increased S-bacillithiolations under HOSCN in S. aureus, supporting its thiol-specific mode of action [1]. Using phenotype analyses, the HOSCN reductase MerA was shown to confer the highest resistance towards HOSCN stress in S. aureus [1, 2]. The crystal structure of the MerA dimer revealed its function as group I flavin disulfide reductase, with the FAD cofactor close to the C43XXXXC48 motif, which interacts with the H427XXXXE432 motif of the opposing subunit, facilitating deprotonation of the active site Cys43 in the HOSCN reduction cycle [2]. Summary: HOSCN causes a thiol-specific stress response, EBSH changes and S-bacillithiolation in S. aureus. MerA functions as HOSCN reductase and main protection mechanism against neutrophil oxidants.

References:

[1] Loi VV, Busche T, Schnaufer F, Kalinowski J, Antelmann H. 2023. The neutrophil oxidant hypothiocyanous acid causes a thiol-specific stress response and an oxidative shift of the bacillithiol redox potential in Staphylococcus aureus. Microbiol Spectr. 11:e0325223

[2] Shearer HL, Loi VV, Weiland P, Bange G, Altegoer F, Hampton MB, Antelmann H, Dickerhof N. 2023. MerA functions as a hypothiocyanous acid reductase and defense mechanism in Staphylococcus aureus. Mol Microbiol. 119:456-470.