Ammonia assimilation and glutamate formation in the anaerobe Selenomonas ruminantium

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Abstract
S. ruminantium possessed 2 pathways for NH4+ assimilation that resulted in net glutamate synthesis. One pathway fixed NH4+ through the action of a NADPH-linked glutamate dehydrogenase (GDH). Maximal GDH activity required KCl (about 0.48 M), but a variety of monovalent salts could replace KCl. Complete substrate saturation of the enzyme by NH4+ did not occur, and apparent Km values of 6.7 and 23 mM were estimated. Also, an NADH-linked GDH activity was observed but was not stimulated by KCl. Cells grown in media containing non-growth-rate-limiting concentrations of NH4+ had the highest levels of GDH activity. The 2nd pathway fixed NH4+ into the amide of glutamine by an ATP-dependent glutamine synthetase (GS). The GS did not display .gamma.-glutamyl transferase activity, and no evidence for an adenylylation/deadenylylation control mechanism was detected. GS activity was highest in cells grown under N limitation. Net glutamate synthesis from glutamine was effected by glutamate synthase activity (GOGAT). The GOGAT activity was reductant dependent, and maximal activity occurred with dithionite-reduced methyl viologen as the source of electrons, although NADPH or NADH could partially replace this artificial donor system. FAD, FMN, or ferredoxin could not replace methyl viologen. GOGAT activity was maximal in cells grown with NH4+ as sole N source and decreased in media containing Casamino Acids.