Assimilation of Ammonia by Bacteroides amylophilus in Chemostat Cultures

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Abstract
The size of the amino acid pool in B. amylophilus, a rumen bacterium, was limited by the availability of NH3 in the growth medium. Alanine was the major pool constituent irrespective of the growth-limiting nutrient. In steady-state NH3-limited chemostat cultures, glutamine synthetase (GS) activity was 10-fold higher and glutamate dehydrogenase (GDH) activity up to 5-fold lower than in maltose-limited cultures. No glutamate synthase (GOGAT) activity was demonstrated. When excess NH3 was pulsed into an NH3-limited chemostat culture, the glutamine pool expanded rapidly and GS was inactivated. This was consistent with the observation that, in a number of bacteria, GS functions to assimilate NH3 when the prevailing concentration is low. GDH was always very active in extracts of B. amylophilus and its Km for NH3 was relatively low (1-2 mM). This suggested that GDH could continue to function biosynthetically when GS was depressed.