Part of a bifunctional enzyme complex that functions as a hydrogen-evolving hydrogenase with sulfur-reducing activity. May play a role in hydrogen cycling during fermentative growth. Activity exhibited with NAD in addition to NADPH. The beta and gamma subunits form the sulfur-reducing component that catalyzes the cytoplasmic production of hydrogen sulfide in the presence of elemental sulfur. H2 + n sulfur = H(+) + hydrogen sulfide + (n-1) sulfur Binds 2 [4Fe-4S] clusters. Measured for the whole complex. Optimum temperature is greater than 90 degrees Celsius. Activity increases with increasing temperature from 30 degrees Celsius to 90 degrees Celsius. Has a half-life of 6 hours at 95 degrees Celsius. Dimer of heterotetramer of alpha, beta, gamma and delta subunits. The nickel-containing alpha and delta subunits constitute the hydrogenase activity. The beta and gamma subunits (flavin-containing dimer) constitute the sulfur reductase activity.