A Potent Quinone Reductase Encoded by ywqN (Qnr1) Protects Bacillus subtilis from Oxygen Radical Genotoxicity

<i>ywqN</i> encodes a protein with an unassigned function that shares partial 3D homology with <i>B. subtilis</i> YhdA, <i>Pseudomonas putida</i> ChrR, and <i>Escherichia coli</i> YieF, which are NADP(H)/FMN-dependent oxidoreductases that catalyze the reduction of diverse chemical pollutants, including Cr(VI). Here, we report that a recombinant His₆-YwqN protein displays marginal chromate reductase activity but is capable of reducing synthetic azo dyes. Remarkably, His₆-YwqN exhibits a potent quinone reductase activity, catalyzing the reduction of menadione (MD) and 1,4-naphthoquinone (NQ). The individual and combined roles of YwqN and YhdA in protecting <i>B. subtilis</i> from ROS-promoting agents were further tested. Sensitization to the oxidizing agent H₂O₂ required the simultaneous loss of both YwqN and YhdA. In contrast, strains deficient in <i>ywqN</i>, either alone or in combination with <i>yhdA</i>, exhibited similar but higher susceptibilities to the superoxide-generating agent MD compared with the WT strain. These results indicate that YwqN and YhdA contribute to protection against the deleterious effects of ROS in <i>B. subtilis.</i> Further results revealed that while YwqN, but not YhdA, prevented MD-induced mutagenesis, both proteins synergistically prevented Rif^R mutations induced by H₂O₂. Furthermore, overexpression of YwqN suppressed the hypermutagenesis phenotype of a <i>B. subtilis</i> strain deficient in the prevention/repair oxidized guanine (GO) system, which is prone to accumulate 8-oxoGs. In summary, YwqN counteracts the cytotoxic and genotoxic effects promoted by ROS in <i>B. subtilis</i> and represents a potential tool for the remediation of soils and effluents contaminated with carcinogenic azo dyes.