Mechanism of H2S Oxidation by the Dissimilatory Perchlorate-Reducing Microorganism italic toggle="yes">Azospira suillum /italic> PS

ABSTRACT The genetic and biochemical basis of perchlorate-dependent H2S oxidation (PSOX) was investigated in the dissimilatory perchlorate-reducing microorganism (DPRM) Azospira suillum PS (PS). Previously, it was shown that all known DPRMs innately oxidize H2S, producing elemental sulfur (So). Although the process involving PSOX is thermodynamically favorable (ΔG°′ = −206 kJ ⋅ mol−1 H2S), the underlying biochemical and genetic mechanisms are currently unknown. Interestingly, H2S is preferentially utilized over physiological electron donors such as lactate or acetate although no growth benefit is obtained from the metabolism. Here, we determined that PSOX is due to a combination of enzymatic and abiotic interactions involving reactive intermediates of perchlorate respiration. Using various approaches, including barcode analysis by sequencing (Bar-seq), transcriptome sequencing (RNA-seq), and proteomics, along with targeted mutagenesis and biochemical characterization, we identified all facets of PSOX in PS. In support of our proposed model, deletion of identified upregulated PS genes traditionally known to be involved in sulfur redox cycling (e.g., Sox, sulfide:quinone reductase [SQR]) showed no defect in PSOX activity. Proteomic analysis revealed differential abundances of a variety of stress response metal efflux pumps and divalent heavy-metal transporter proteins, suggesting a general toxicity response. Furthermore, in vitro biochemical studies demonstrated direct PSOX mediated by purified perchlorate reductase (PcrAB) in the absence of other electron transfer proteins. The results of these studies support a model in which H2S oxidation is mediated by electron transport chain short-circuiting in the periplasmic space where the PcrAB directly oxidizes H2S to So. The biogenically formed reactive intermediates (ClO2− and O2) subsequently react with additional H2S, producing polysulfide and So as end products. IMPORTANCE Inorganic sulfur compounds are widespread in nature, and microorganisms are central to their transformation, thereby playing a key role in the global sulfur cycle. Sulfur oxidation is mediated by a broad phylogenetic diversity of microorganisms, including anoxygenic phototrophs and either aerobic or anaerobic chemotrophs coupled to oxygen or nitrate respiration, respectively. Recently, perchlorate-respiring microorganisms were demonstrated to be innately capable of sulfur oxidation regardless of their phylogenetic affiliation. As recognitio