SUMMARY: Ammonia assimilation was studied using continuous cultures of three obligate methanotrophs. The type X organism, (Bath), assimilated ammonia during growth on dinitrogen or nitrate via the glutamine synthetase/glutamate synthase pathway but utilized the alanine dehydrogenase pathway when grown in the presence of excess ammonia. Repression and derepression of these ammonia assimilation enzymes was demonstrated during the switchover of continuous cultures from nitrogen-free (N-fixing) medium to medium containing high concentrations of ammonia. The properties of alanine dehydrogenase and glutamate synthase in this organism are discussed. In the type I methanotroph, S1, the pathway of ammonia assimilation was again dependent on the source of fixed nitrogen in the growth medium, but in the type II methanotroph, OB3b, ammonia was assimilated exclusively via the glutamine synthetase/glutamate synthase pathway.


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