The chemolithoautotrophic archaeon Pyrodictium abyssi isolate TAG 11 gains energy by reducing sulfur with H 2 to H 2 S. From this hyperthermophile, a sulfur-reducing complex catalyzing this reaction was purified 13.5-fold. The native complex exhibited a brownish-yellow colour and showed an apparent molecular mass of 520 kDa. SDS/PAGE revealed the presence of nine different major polypeptides with apparent molecular masses of 82, 72, 65, 50, 47, 42, 40, 30 and 24 kDa. The native complex contained 50Ϫ55 mol acid-labile sulfur, 50Ϫ55 mol iron, 1.6 mol nickel, 1.2 mol copper, 2.8 mol cytochrome b and 0.3 mol cytochrome c (all per mol native complex). The temperature optimum of the H 2 :sulfur oxidoreductase complex was 100°C, which is consistent with the physiological growth optimum of the native organism. The complex is extremely heat stable. During 5 h incubation at 100°C, no decrease in H 2 Sforming activity could be observed.Keywords : oxidoreductase ; membrane bound ; sulfur respiration; hyperthermophilic ; Pyrodictium abyssi.Based on the 16S rRNA sequences, the deepest and shortest al., 1995). Reduction of S°or polysulfides is mediated by a membrane-bound polysulfide reductase, a molybdo-enzyme branches of the universal phylogenetic tree represent hyperthermophiles (Stetter, 1992). Most of them belong to the archaeal consisting of three subunits (Krafft et al., 1992). The mechanism of electron transport from the formate dehydrogenase and hydrodomain (Woese et al., 1990). In the metabolism of many hyperthermophilic Archaea, sulfur plays an important role. The obli-genase to the polysulfide reductase is not clear (Schröder et al., 1988;Krafft et al., 1995). Here, we report the purification and gate heterotrophic strains of Pyrococcus and Thermococcus grow on organic compounds (Fiala and Stetter, 1986; Zillig et characterization of the first extremely thermostable membranebound sulfur-reducing enzyme complex isolated from the al., 1983; Neuner et al., 1990). In the presence of sulfur or polysulfides, H 2S is formed (Neuner et al., 1990; Blumentals et al., chemolithoautotrophic hyperthermophile, Pyrodictium abyssi isolate TAG 11. It is proposed to contain the entire electron-1990; Keller et al., 1995). From Pyrococcus furiosus, a cytosolic sulfide dehydrogenase which uses NADPH to reduce polysul-transport chain required for the reduction of S°with H 2 to H 2 S, including a hydrogenase, a sulfur reductase and electronfides to H 2S (Ma and Adams, 1994), and a cytosolic hydrogenase with sulfur reductase activity are known. transfering components. In this organism, the presence of sulfur increases growth yields (Schicho et al., 1993). However, so far there is no evidence that MATERIALS AND METHODS sulfur reduction in P. furiosus is coupled with ATP synthesis via Growth of organisms. The isolate P. abyssi TAG 11 was sulfur respiration (Schönheit and Schäfer, 1995). Chemolithoaucultivated on elemental sulfur or thiosulfate as described pretotrophic archaea, as Pyrodictium, utilize the redox couple H2/ viously (Stetter...