Regulation of Ammonia Assimilation

Heterotrophic bacteria like Escherichia coli and Klebsiella aerogenes induce the operation of GS-glutamate synthase pathway of ammonia assimilation under conditions of low ammonia and of GDH-pathway under conditions of high ammonia. Thus the two pathways are mutually exclusive and the level of cellular ammonia determines which pathway of ammonia assimilation is likely to operate.

Cyanobacteria like systems have only Gs-glutamate synthase pathway functioning under low or high cellular ammonia. Information about the role of ammonia in metabolic regulation of its two assimilatory pathways in higher plants is virtually unknown. Animals get rid of excess ammonia through the urea cycle in order to escape ammonia toxicity. Plants have no similar mechanism of escaping ammonia toxicity. This might be because land plants evolved under conditions of nitrogen limitation. However plants seem to have developed controls of nitrogen metabolism which ensure that ammonia is reassimilated.