Characterization and Regulation of Sulfur Reductase Activity in Thermotoga neapolitana

The growth of the hyperthermophilic, anaerobic bacterium Thermotoga neapolitana is stimulated by elemental sulfur by an unknown mechanism. We detected hydrogen-dependent sulfur reductase (sulfhydrogenase) and polysulfide dehydrogenase activities in cell extracts of this organism, demonstrating that it has at least two pathways for sulfidogenesis. Hydrogen-dependent sulfur reductase and hydrogenase activities are catalyzed by the purified hydrogenase of Thermotoga maritima, and this enzyme was called the sulfhydrogenase (K. Ma, R. N. Schicho, R. M. Kelly, and M. W. W. Adams, Proc. Natl. Acad. Sci. USA 90:5341-5344, 1993). Cells grown without elemental sulfur or cystine had 1.3 to 3.3 times higher sulfhydrogenase activities than those grown with either of these sources of sulfane sulfur. Hydrogenase activity was 2 to 5 times higher. Polysulfide dehydrogenase was up to 48-fold more active in cell extracts than the sulfhydrogenase. The activity of polysulfide dehydrogenase was approximately twofold higher when cells were grown in the presence of elemental sulfur. Its activity was oxygen labile in crude extracts, and it appears to be a cytoplasmic enzyme. Polysulfide was preferred over elemental sulfur as an electron acceptor (K_m = 0.15 mM) and was more active with NADH (K_m = 0.03 mM) than NADPH (K_m = 0.41 mM). Growth in the presence of elemental sulfur appeared to slightly increase the activity of polysulfide dehydrogenase and slightly decrease both activities of sulfhydrogenase (hydrogenase and polysulfide reductase), while growth without elemental sulfur had the opposite effects. The greater activity of polysulfide dehydrogenase and its apparent regulation indicate that it is the more physiologically important means of polysulfide reduction.