Compartmentation of Sulfur Metabolites in Tobacco Cells

The distribution of sulfur-containing metabolites in cultured tobacco cells was determined by analyzing efflux kinetics. Transported sulfate rapidly labeled the cytoplasmic pools of sulfate (1 hour) and sulfur amino acids (6 hours). Excess sulfate and amino acids were transported into the vacuole. The size and distribution of the amino acid pool was not affected by increasing the sulfate content of the cells.Transported cysteine was rapidly degraded to sulfate. The cytoplasmic pools of sulfate and amino acids were completely labeled within 6 hours, whereas the vacuolar pools were labeled more slowly. The intracellular cysteine pool was elevated by decreasing the pH of the transport medium. In all experiments, between 60 and 90% of the label was present in the vacuole.