Isolation and identification of an iron-oxidizing bacterium which can grow on tetrathionate medium and the properties of a tetrathionate- decomposing enzyme isolated from the bacterium

Among 150 pure strains of iron-oxidizing bacteria obtained from natural environments, two strains, Funis 2-1 and OK1-50, had the ability to use potassium tetrathionate (K₂S₄O₆) as a sole energy source for growth. Funis 2-1 was a gram-negative, rod-shaped, acidophilic iron- and sulfur-oxidizing chemolithotrophic bacterium and had the same cytochrome composition and mean G+C content of DNA as Thiobacillus ferrooxidans, indicating that the strain is T. ferrooxidans. A tetrathionate-decomposing enzyme that catalyzes the disproportionate metabolism of 4 tool of tetrathionate into 7 mol of thiosulfate and 2 tool of sulfate was located on the plasma membrane of K₂S₄O₆-grown, but not Fe²⁺-grown Funis 2-1 cells. Washed intact cells and cell extracts prepared from Funis 2-1 cells grown on K₂S₄O₆ medium supplemented with more than 11 mM FeSO₄ did not show K₂S₄O₆-decomposing enzyme activity. K₂S₄O₆-decomposing enzyme was purified to homogeneity from K₂S₄O₆-grown Funis 2-1 cells. The apparent molecular weight of this enzyme was estimated to be 50,000 by gel filtration, 50,000 by SDS-PAGE, and 49,600 using a time-of-flight mass spectrometer, indicating that the enzyme is monomeric. The enzyme was most active at pH 3.5 and 50°C and the activity was enhanced approximately 18 fold by a concentration of 200 mM of sulfate ion. The Michaelis constant of this enzyme for K₂S₄O₆ was 0.73 mM.