Abstract
Summary: The size of the amino acid pool in Bacteroides amylophilus, a rumen bacterium, was limited by the availability of ammonia in the growth medium. Alanine was the major pool constituent irrespective of the growth-limiting nutrient. In steady-state ammonia-limited chemostat cultures, glutamine synthetase (GS) activity was tenfold higher and glutamate dehydrogenase (GDH) activity up to fivefold lower than in maltose-limited cultures. No glutamate synthase (GOGAT) activity was demonstrated. When excess ammonia was pulsed into an ammonia-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 ammonia when the prevailing concentration is low. However, GDH was always very active in extracts of B. amylophilus and its Michaelis constant for ammonia was relatively low (1 to 2 mm). This suggested that GDH could continue to function bio-synthetically when GS was derepressed.
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