<i>Staphylococcus aureus sqr</i> Encodes a Type II Sulfide:Quinone Oxidoreductase and Impacts Reactive Sulfur Speciation in Cells

Recent studies implicate hydrogen sulfide (H₂S) oxidation as an important aspect of bacterial antibiotic resistance and sulfide homeostasis. The <i>cst</i> operon of the major human pathogen <i>Staphylococcus aureus</i> is induced by exogenous H₂S stress and encodes enzymes involved in sulfide oxidation, including a group I flavoprotein disulfide oxidoreductase sulfide:quinone oxidoreductase (SQR). In this work, we show that <i>S. aureus</i> SQR catalyzes the two-electron oxidation of sodium sulfide (Na₂S) into sulfane sulfur (S⁰) when provided flavin adenine dinucleotide and a water-soluble quinone acceptor. Cyanide, sulfite, and coenzyme A (CoA) are all capable of functioning as the S⁰ acceptor <i>in vitro</i>. This activity requires a C167–C344 disulfide bond in the resting enzyme, with the intermediacy of a C344 persulfide in the catalytic cycle, verified by mass spectrometry of sulfide-reacted SQR. Incubation of purified SQR and <i>S. aureus</i> CstB, a known Fe^II persulfide dioxygenase-sulfurtransferase also encoded by the <i>cst</i> operon, yields thiosulfate from sulfide, in a CoA-dependent manner, thus confirming the intermediacy of CoASSH as a product and substrate of SQR and CstB, respectively. Sulfur metabolite profiling of wild-type, Δ<i>sqr</i>, and Δ<i>sqr</i>::pSQR strains reveals a marked and specific elevation in endogenous levels of CoASSH and inorganic tetrasulfide in the Δ<i>sqr</i> strain. We conclude that SQR impacts the cellular speciation of these reactive sulfur species but implicates other mechanisms not dependent on SQR in the formation of low-molecular weight thiol persulfides and inorganic polysulfides during misregulation of sulfide homeostasis