When cultured tobacco cells are provided growth-limiting concentrations of sulfur as sulfate, the rate of development of nitrate reductase (NADH:nitrate oxidoreductase, EC 1.6.6.1) is proportional to the initial sulfate concentration. When the cells are provided growth-limiting concentrations of nitrogen as nitrate, the rate of derepression of ATP sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4) is proportional to the initial nitrate concentration. These results are taken to be indicative of a reciprocal regulatory coupling between the nitrate and sulfate assimilation pathways.

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