Bacillus sp. STG-83 that was isolated from Neidasht spring - north of Iran - is a grain positive faculative anaerobic rod which is capable of reducing selenate anion to elemental selenium. This bacterium is also able to catalyze the reduction of tellurite. The chromatographic behavior demonstrated that the reduction of tellurite and selenate in Bacillus sp. STG-83 was carried out by two distinct enzymes. Both enzymes were purified by the same purification procedure. The purification process involved (NH4)(2)SO4 precipitation, followed by phenyl-Sepharose and Sephacryl S-200 chromatography. Using gel filtration, the relative molecular mass of the native selenate reductase and tellurite reductase was determined to be 182 kDa and 197 kDa, respectively. Values of 2.7 mu mol min(-1) mg(-1) and 3.24 mM were obtained for the V-max and K-m of selenate reductase, respectively. The K-m for selenite reduction was determined to be 10.9 mM and the V-max was 1.6 mu mol min(-1) mg(-1) of protein. For tellurite the K-m was 2.63 mM and the V-max was 5.2 mu mol min(-1) mg(-1) of protein. Optimal pH range for selenate reductase was 5.5-6, and the maximum activity was achieved at 38 degrees C. Tellurite reductase exhibited its highest activity at 35 C and the optimum pH of the purified enzyme was 8. Cations Such as Ca2+, Mg2+ and Na+ abolished the selenate reductase activity and had no effect on the tellurite reductase activity. Tellurite reductase was inactivated by EDTA. Elemental analysis revealed the presence of Zn and Mo in tellurite reductase and Cu in selenate reductase. (C) 2009 Elsevier Inc. All rights reserved.