Ammonium (methylammonium) is the co‐repressor of nitrate reductase in Chlamydomonas reinhardii

Methylammonium cannot be used as a nitrogen source by the green alga Chlamydomonas reinhardii and, like ammonia, caused the repression of nitrate reductase without affecting the photosynthetic activity. Glutamine synthetase catalyzed the conversion ofmethylammonium to a single product, identified as γ‐ N ‐methylglutamine, which accumulated in the cells. Derepression of nitrate reductase was accompanied by a decrease in the intracellular concentration of methylammonium and a concomitant accumulation of γ‐ N ‐methylglutamine inside the cells. These facts strongly suggest that ammonium (methylammonium) per se, and not a product of its metabolism, is the co‐repressor of nitrate reductase in C. reinhardii