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When Thiobacillus jerrooxidans AP19-3 was incubated in sulphur-salts medium with 1 mM- Co2+, the sulphur : ferric-ion oxidoreductase (SFORase) of washed intact cells completely disappeared and a concomitant cessation of cell growth was observed. However, when reduced glutathione (GSH), which is absolutely required for SFORase activity, was added to the cells seemingly lacking SFORase activity, the activity was completely restored. The total GSH content of the cells incubated with or without Co2+ was 0.10 and 0.16pmol (mg protein)-', respectively. The SFORase activity of cell-free extracts in the presence of added GSH was 74% of the whole cell activity without Co*+, indicating that an active SFORase was still present but that the GSH required for SFORase activity was in short supply after incubating the cells in sulphur-salts medium with Co2+. Incubating SFORase with 1 mM-Co2+ did not decrease its activity, whereas incubating with Co2+ plus GSH markedly decreased activity. Sulphite (1 mM), one of the products of sulphur oxidation by SFORase, partially restored this loss of SFORase activity. A new type of mechanism for the inhibition by Co2+ of the sulphur metabolism of T. ferrooxidans is proposed : Co2+ stops cell growth on sulphur by decreasing the intracellular GSH concentration to a level at which SFORase is no longer active, and the cells then cannot obtain energy by oxidizing elemental sulphur.

Mechanism of Inhibition by Co2+ of the Growth of Thiobacillus ferrooxidans on Sulphur-Salts Medium