Abstract
The utilization of ammonium by Chlamydomonas is mainly a light-dependent process, mediated by the glutamine synthetase-glutamate synthase pathway. Ferredoxin-dependent and NADH-glutamate synthase activities were measured in Chlamydomonas cells treated with toluene. The assay with reduced methyl viologen as electron donor measures the sum of the two activities. The Fd- and NADH-glutamate synthase activities respond differently to several nutritional conditions consistent with different physiological roles for the two enzymes. The Fd-glutamate synthase is probably responsible for the photosynthetic assimilation of ammonium and the reassimilation of ammonium formed during photorespiration. On the other hand, the NADH-glutamate synthase may be involved more in the assimilation of ammonium in the dark and in the recycling of ammonium released by protein metabolism under low availability of nitrogen.
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