Pathway of ammonia assimilation in illuminated and darkened Chlamydomonas reinhardii

Evidence is presented which shows that NH3 assimilation in Chlamydomonas occurs exclusively via the glutamate synthase cycle in illuminated and darkened cells and those in which the internal level of NH3 is elevated. This result indicates that glutamate dehydrogenase probably plays a catabolic rather than anabolic role in the N nutrition of the alga. Glutamine synthetase and glutamate dehydrogenase were characterized and their kinetic properties shown to be consistent with these proposals. It is suggested that reversible activity modulations of glutamine synthetase regulate the operation of the glutamate synthase cycle in the light but the availability of reductant and ATP limits its activity in darkened cells. The possible involvement of the two glutamate synthase enzymes in both light and dark assimilation is discussed.