Regulation of Sulfate Assimilation in Plants

When 0.5 mm cysteine is added to cultures of Lemna minor L. growing with sulfate as the sole sulfur source, there is a rapid 80% loss of extractable adenosine 5'-phosphosulfate sulfotransferase. This loss is accompanied by an inhibition of sulfate uptake; however, lack of sulfate is not responsible for the decreasing adenosine 5'-phosphosulfate sulfotransferase activity.Cultivation with cysteine causes an increase in the cyst(e)ine pool of L. minor. This fact taken together with the observed inactivation of adenosine 5'-phosphosulfate sulfotransferase in crude extracts by cysteine suggests that the cysteine pool is involved in the in vivo regulation of the enzyme.The activity of adenosine 5'-phosphosulfate sulfotransferase is restored within 24 hours after transfer to a culture medium without cysteine. This restoration is partially blocked by 6-methyl purine and actinomycin D and completely by cycloheximide.Cycloheximide added to cultures of L. minor L. causes a loss of extractable APSTase comparable to the one obtained with cysteine. This loss may be in part due to cysteine, since cycloheximide causes a pronounced increase in the cysteine pool of L. minor.