GLUTAMINE-SYNTHETASE ACTIVITY, AMMONIA ASSIMILATION AND CONTROL OF NITRATE REDUCTION IN THE UNICELLULAR RED ALGA CYANIDIUM-CALDARIUM

Addition of l-methionine-dl-sulphoximine to cells of Cyanidium caldarium brings about a loss of glutamine synthetase activity. Concomitantly ammonia assimilation is prevented. Under physiological conditions nitrate reductase [NAD(P)H: nitrate oxidoreductase EC 1.6.6.2] is reversibly converted into an inactive enzyme upon addition of ammonia. In the presence of methionine sulphoximine, when glutamine synthetase activity is lost, nitrate reductase is no longer inactivated by ammonia. It is suggested that ammonia itself is not the actual effector of nitrate reductase inactivation. Concomitantly with the failure of nitrate reductase to undergo ammonia-inactivation, in the presence of methionine sulphoximine nitrate reduction is an uncontrolled process, thus, in media with nitrate ammonia continues to be produced and excreted into the external medium at a constant rate. © 1979 Springer-Verlag.