The Metabolism of Thiosulfate in Salmonella Typhimurium.

Inorganic thiosulfate has been implicated as an intermediate in the assimilation of sulfate in a variety of organisms. Nutritional studies with mutants of Aspergillus have been interpreted to indicate thiosulfate as an intermediate between sulfite and cysteine (1, 2), possibly via a pathway involving cysteine-Ssulfonate as a direct precursor of cyst&e (24). Shepherd earlier had envisioned the utilization of thiosulfate by this organism as involving a pathway distinctly separate from that explaining the utilization of sulfate and sulfite (5). The nutritional behavior of Neurospora mutants suggested inorganic thiosulfate (6), or an organic derivative thereof (7), to be an intermediate in the reduction of sulfite to sulfide. The ability of thiosulfate to reduce the incorporation of isotopically labeled sulfate by Escherichia coli (8) and by Salmonella typhimurium (9) suggested the ion or an immediate derivative as a direct intermediate in sulfate incorporation in these bacteria. Similar studies with yeast (10, 11) and with Desuljovibrti (12) tended to rule out thiosulfate as an intermediate in sulfate metabolism in these organisms. The reduction of thiosulfate has been reported in cell-free preparations from yeast (13), from DesuZjovibrio desuljuricans (14-16), from Proteus (17), from E. coli (I@, from Thiobacillus thioparus (19), and from Micrococcus Zactilyticus (20) by use of a variety of reducing agents. Sulfide, and in certain cases sulfite as well (13, l&20), was recognized as a product of the reduction. Recent studies in this laboratory have been concerned with the properties of a large collection of mutants of Salmonella typhimurium which have in common a nutritional sulfur requirement that cannot be met by inorganic sulfate but is satisfied by the inclusion of cysteine in the growth medium (21). Within this collection are three classes of mutants which lack the ability to reduce inorganic sulfite but retain the ability to utilize thiosulfate as a sole sulfur source. A preliminary investigation revealed that these latter mutants would achieve but half the growth yield on thiosulfate which was characteristic of wild-type Salmonella or of mutants retaining the ability to utilize inorganic sulfite.1 This observation suggested the nonutilization of 1 of the sulfur atoms of thiosulfate by cells lacking sulfite reductase, and prompted the present investigations. Evidence will be presented to demonstrate that thiosulfate metabolism in this organism may be pictured as an alternate pathway which converges with that for the assimilation of sulfate into cysteine, and that