Ammonium regulation of cellular nitrate and nitrite reductases (NR;NiR) was studied in Phormidium uncinatum. Diminished to low activities were found with ammonia and development of the enzymes in its absence required de novo protein synthesis but not nitrate. While an inhibition of ammonium metabolism via glutamine synthetase (GS) activity reversed the ammonium effect on NR. NiR was non-responsive. Development of NiR in such cultures in absence of ammonia however needed new protein synthesis. These data suggest that ammonia by itself and through assimilation exerts repression control on NiR and NR respectively, which are otherwise derepressed. © 1990 Federation of European Microbiological Societies.
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