Regulation of the ammonia assimilatory enzymes in Salmonella typhimurium
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Abstract:
The regulation of glutamate dehydrogenase (EC 1.4.1.4), glutamine synthetase (EC 6.3.1.2), and glutamate synthase (EC 2.6.1.53) was examined for cultures of Salmonella typhimurium grown with various nitrogen and amino acid sources. In contrast to the regulatory pattern observed in Klebsiella aerogenes, the glutamate dehydrogenase levels of S. typhimurium do not decrease when glutamine synthetase is derepressed during growth with limiting ammonia. Thus, it appears that the S. typhimurium glutamine synthetase does not regulate the synthesis of glutamate dehydrogenase as reported for K. aerogenes. The glutamate dehydrogenase activity does increase, however, during growth of a glutamate auxotroph with glutamate as a limiting amino acid source. The regulation of glutamate synthase levels is complex with the enzyme activity decreasing during growth with glutamate as a nitrogen source, and during growth of auxotrophs with either glutamine or glutamate as limiting amino acids.Keywords:
Glutamate synthase
Glutamic acid
Glutaminase
Glutamate synthase
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The enzymes involved in the assimilation of ammonia by free-living cultures of Rhizobium spp. are glutamine synthetase (EC. 6.o.I.2), glutamate synthase (L-glutamine:2-oxoglutarate amino transferase) and glutamate dehydrogenase (ED I.4.I.4). Under conditions of ammonia or nitrate limitation in a chemostat the assimilation of ammonia by cultures of R. leguminosarum, R. trifolii and R. japonicum proceeded via glutamine synthetase and glutamate synthase. Under glucose limitation and with an excess of inorganic nitrogen, ammonia was assimilated via glutamate dehydrogenase, neither glutamine synthetase nor glutamate synthase activities being detected in extracts. The coenzyme specificity of glutamate synthase varied according to species, being linked to NADP for the fast-growing R. leguminosarum, R. melitoti, R. phaseoli and R. trifolii but to NAD for the slow-growing R. japonicum and R. lupini. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase activities were assayed in sonicated bacteroid preparations and in the nodule supernatants of Glycine max, Vicia faba, Pisum sativum, Lupinus luteus, Medicago sativa, Phaseolus coccineus and P. vulgaris nodules. All bacteroid preparations, except those from M. sativa and P. coccineus, contained glutamate synthase but substantial activities were found only in Glycine max and Lupinus luteus. The glutamine synthetase activities of bacteroids were low, although high activities were found in all the nodule supernatants. Glutamate dehydrogenase activity was present in all bacteroid samples examined. There was no evidence for the operation of the glutamine synthetase/glutamate synthase system in ammonia assimilation in root nodules, suggesting that ammonia produced by nitrogen fixation in the bacteroid is assimilated by enzymes of the plant system.
Glutamate synthase
Nitrogen Assimilation
Rhizobium leguminosarum
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Glutamate synthase
Derepression
Adenylylation
Nitrogen Assimilation
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Glutamate synthase
Nitrogen Assimilation
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Glutamate synthase
Nitrogen Assimilation
Oxidative deamination
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Levels of activity of glutamine synthetase, glutamate dehydrogenase and NADH-dependent glutamate synthase in nodule cytoplasm extracts of twelve herbaceous legume species have been measured. Nodules of all species contained substantial quantities of glutamine synthetase. Levels of glutamate synthase were found to be between 7 and 100% of those of glutamine synthetase, while levels of glutamate dehydrogenase varied widely between 0.2 and 150% of those of glutamine synthetase. The estimated Km for hydroxylamine of glutamine synthetase was found to vary between 0.02 and 0.5 mM in nine species tested, while that of glutamate dehydrogenase for ammonia varied between 0.03 M and 0.1 M in the four species containing significant levels of that enzyme. It is proposed that the pathway of ammonia assimilation via glutamine synthetase and NADH-dependent glutamate synthase-catalysed reactions is universal in legume nodule metabolism.
Glutamate synthase
Nitrogen Assimilation
Hydroxylamine
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The activities of the following enzymes were studied in connection with dinitrogen fixation in pea bacteroids: glutamine synthetase(L-glutamate: ammonia ligase (ADP-forming)(EC 6.3.1.2)(GS); glutamate dehydrogenase (NADP+)(L-glutamate: NADP+ oxidoreductase (deaminating)(EC 1.4.1.4)(GDH); glutamate synthase (L-glutamine: 2-exeglutarate aminotransferase (NADPH-oxidizing))(EC 2.6.1.53)(GOGAT). GS activity was high throughout the growth of the plant and GOGAT activity was always low. It is unlikely that GDH or the GS-GOGAT pathway can account for the incorporation of ammonia from dinitrogen fixation in the pea bacteroid,
Glutamate synthase
Nitrogen Assimilation
Rhizobium leguminosarum
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Glutamate synthase
Nitrogen Cycle
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SUMMARY: In order to explore the function of each of the four enzymes involved in ammonium assimilation in Neurospora crassa [NADP-dependent glutamate dehydrogenase (GDH), NADH-dependent glutamate synthase (GOGAT) and the α and β isozymes of glutamine synthetase (GS)] growth curves, enzymic activities and intracellular pools of ammonium, glutamate and glutamine were determined in the wild-type as well as in mutants lacking one or two of these biosynthetic enzymes. These parameters were measured in ammonium-limited chemostat-type cultures, in which conditions resembling a continuous culture were achieved for the first time for a filamentous fungus, and in ammonium-excess batch cultures. NADP-dependent GDH appeared to be the main provider of cellular glutamate in the presence of limited and excess ammonium but could be partially replaced by NADH-dependent GOGAT depending upon the presence of GSβ. GOGAT appeared to have the main role in the recycling of glutamine to glutamate, particularly in ammonium-limited conditions. GSβ was the main provider of cellular glutamine but GSα could substitute for this activity if GDH was also present. In conditions of ammonium excess GDH and GOGAT were repressed by glutamate and GS was repressed by glutamine, as previously found. However, in conditions of ammonium limitation these enzymes appeared to escape nitrogen repression.
Glutamate synthase
Chemostat
Neurospora
Nitrogen Assimilation
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Glutamate synthase
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