Characterization of tetrathionate hydrolase from the marine acidophilic sulfur-oxidizing bacterium, <i>Acidithiobacillus thiooxidans</i> strain SH

Tetrathionate hydrolase (4THase), a key enzyme of the S₄-intermediate (S4I) pathway, was partially purified from marine acidophilic bacterium, <i>Acidithiobacillus thiooxidans</i> strain SH, and the gene encoding this enzyme (SH-<i>tth</i>) was identified. SH-Tth is a homodimer with a molecular mass of 97 ± 3 kDa, and contains a subunit 52 kDa in size. Enzyme activity was stimulated in the presence of 1 M NaCl, and showed the maximum at pH 3.0. Although 4THases from <i>A. thiooxidans</i> and the closely related <i>Acidithiobacillus caldus</i> strain have been reported to be periplasmic enzymes, SH-Tth seems to be localized on the outer membrane of the cell, and acts as a peripheral protein. Furthermore, both 4THase activity and SH-Tth proteins were detected in sulfur-grown cells of strain SH. These results suggested that SH-Tth is involved in elemental sulfur-oxidation, which is distinct from sulfur-oxidation in other sulfur-oxidizing strains such as <i>A. thiooxidans</i> and <i>A. caldus</i>. SH-Tth from the marine bacterium exhibited halophilic features distinct from those of the limnetic <i>Af-</i>Tth.