Regulation of Sulfate Assimilation in Tobacco Cells: EFFECT OF NITROGEN AND SULFUR NUTRITION ON SULFATE PERMEASE AND O-ACETYLSERINE SULFHYDRYLASE.

The effect of nitrogen and sulfur nutrition on sulfate permease and O-acetylserine sulfhydrylase was studied in tobacco cells.Sulfate transport rates increased 10-fold in cells transferred to sulfur-deficient B-5 medium. The addition of either sulfate or l-cysteine reduced transport 95 and 80%, respectively. The pools of sulfate, cysteine, glutathione, and methionine declined in sulfur-starved cells. The addition of either sulfate or l-cysteine increased the pools of sulfur-containing compounds, but major quantitative differences were measured. Nitrogen-starved cells had low transport rates which were not increased by addition of nitrate/ammonia. The pools of sulfate, cysteine, and methionine were high in nitrogen-starved cells and remained high upon addition of a nitrogen source. The results show that sulfate transport is regulated by the intracellular sulfate pool.O-Acetylserine sulfhydrylase was not affected by sulfur nutrition. The extractable activity was high in B-5-grown cells, sulfur-deficient cells, and cells to which either sulfate or l-cysteine had been added. In contrast, the enzyme declined in cells transferred to nitrogen-deficient medium and the amount of enzyme/g fresh weight increased 10-fold when nitrate/ammonia was added. The addition of nitrate/ammonia had no effect on the cysteine or methionine pools but increased the total amino acid pool. The amount of O-acetylserine was positively correlated with extractable enzyme activity. This enzyme is positively regulated by an effector (possibly O-acetylserine) which is high under conditions of net nitrate assimilation.