Overexpression of sulfide:quinone reductase (SQR) in <i>Acidithiobacillus ferrooxidans</i> enhances sulfur, pyrite, and pyrrhotite oxidation

Hydrogen sulfide is produced during the dissolution of some sulfidic minerals and during the microbial metabolism of reduced sulfur compounds. The sulfide:quinone reductase (SQR) enzyme is able to oxidize H₂S, and the bioleaching cells <i>Acidithiobacillus ferrooxidans</i> have two SQR genes, only one of which has been characterized. We cloned and overexpressed the two SQR genes in <i>A. ferrooxidans</i> and show that they both have SQR activity. Both AFE_0267 and AFE_1792 are active under anaerobic conditions, but only AFE_1792 is active under aerobic conditions. The effect of the SQR overexpression and the expression of related genes on sulfur metabolism was investigated. The overexpression of SQR improved cell growth and sulfur oxidation, suggesting enhanced SQR activity led to a reduction in H₂S toxicity as well as providing additional energy through H₂S oxidation. Additionally, the impact on the oxidation of pyrite and pyrrhotite was investigated. The rate of oxidation of pyrite by the engineered cells was enhanced, and, furthermore, the rate of pyrrhotite oxidation was more than doubled.IMPORTANCEH₂S is a toxic sulfur intermediate, and the SQR enzyme has evolved to oxidize H₂S in <i>A. ferrooxidans</i>. In addition to detoxification, H₂S oxidation provides energy, and overexpression of SQR enhanced aerobic and anaerobic growth on sulfur. The SQR overexpression also enhanced pyrite and pyrrhotite oxidation, which may facilitate the pyrometallurgical processing of a number of critical materials including copper, nickel, and the platinum group metals.