Cysteine does not repress adenosine-5′-phosphosulfate reductase through its conversion to either sulfate or glutathione

Cysteine (Cys) represses the activity of several key regulatory enzymes in the plant sulfate assimilatory pathway. However, it is not clear whether this effect arises from Cys itself or through its conversion to either sulfate or glutathione (GSH). Therefore, we examined this phenomenon by analyzing the activity of adenosine-5′-phosphosulfate (APS) reductase. Both APS reductase (AR) activity and mRNA levels were decreased by treating Arabidopsis thaliana roots with 1 mM Cys. The intracellular sulfate concentration was not affected, whereas enzymatic activity and, to some extent, the mRNA level, declined. Cys treatment in sulfur-starved plants also diminished both parameters. However, this response to Cys was more efficient than when plants were treated with an equal amount of sulfate. When Cys was removed from both Cys- and sulfate-fed plants, AR activity was recovered; the same removal of sulfate was not so effective. Moreover, buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, did not influence the repression of AR by Cys. Finally the AR enzyme was inhibited by cysteine in vitro. These results indicate that Cys represses AR by inhibiting mRNA expression and by directly repressing enzymatic activity, rather than through its conversion to either sulfate or GSH.