HydSL hydrogenase of Thiocapsa bogorovii may participate in sulfur respiration

According to the modern classification of NiFe hydrogenases, the HydSL hydrogenase of Thiocapsa bogorovii is assigned to subgroup 1e, the so-called isp-type hydrogenases. This subgroup is proposed to unite hydrogenases involved in sulfur respiration; however, direct experimental evidence supporting this function remains limited. In this study, we isolated a protein complex containing the HydSL hydrogenase from T. bogorovii that exhibits high activity in the H₂ + S⁰ → H₂S reaction. Under native electrophoresis, the complex migrated as a single band, whereas SDS-PAGE resolved it into seven bands, four of which matched the molecular masses of HydS, Isp1, Isp2, and HydL proteins. Incubation of T. bogorovii cells in a hydrogen atmosphere in darkness and in the presence of elemental sulfur resulted in increased expression of the hydS, hydL, isp1, and isp2 genes encoding the subunits and partner proteins of the complex. The elevated transcript levels correlated with increased cellular activity in the reduction of elemental sulfur to hydrogen sulfide. These observations indicate that the HydSL-containing protein complex may participate in sulfur respiration in T. bogorovii under dark, anaerobic conditions in the absence of oxygen and fermentable substrates. Based on these results and literature data, we propose a putative mechanism for the complex, consistent with its designation as a sulfhydrogenase.