title: Regulation of ammonia assimilation in an obligate methylotroph Methylobacillus flagellatum under steady-state and transient growth conditions.
abstract: The obligate methylotroph Methylobacillus flagellatum was grown in the presence of different ammonium concentrations and the regulation of the enzymes associated with ammonium assimilation was investigated in steady-state and transient growth regimes. As the medium changed from C-limitation to dual C/N- and finally to N-limitation, the culture passed through three definite growth phases. The NADP(+)-dependent glutamate dehydrogenase (GDH) was present under ammonium limitation of the culture growth (at 2 mmol 1(-1) of ammonium in the growth medium) and increased in response to an increase in nitrogen availability. Glutamine synthetase (GS) and glutamate synthase (GOGAT) activities were negligible during C- and C/N-limitation. In N-limited cells the GOGAT activity increased as the dilution rate increased up to 0.35 h-1, and then sharply dropped. In the N-sufficient cultures both NAD(+)-and NADP(+)-dependent isocitrate dehydrogenase (NAD-ICDH and NADP-ICDH) activities were up-regulated as dilution rate increased, but in the N-limited culture the NAD-ICDH activity was up-regulated whereas NADP-ICDH one was down-regulated. Pulse additions of ammonium and methanol demonstrated the coordinate regulation of the GDH and ICDHs activities. When pulses were added to the C/N-limited cultures, there was an immediate utilization of the nutrients, resulting in an increase in biomass; at the same time the GDH and ICDH activities increased and the GS and GOGAT activities decreased. When the same ammonium/methanol pulse was added into the N-limited culture, there was a 3 h delay in the culture response, after which the substrates were utilized at rates close to the ones shown by the C/N-limited culture after the analogous pulse.
authors.affiliation: Institute for Genetics and Selection of Industrial Microorganisms, Moscow, Russia.
authors.forename: M V
authors.initials: MV
authors.lastname: Baev
authors.forename: M Y
authors.initials: MY
authors.lastname: Kiriukhin
authors.forename: Y D
authors.initials: YD
authors.lastname: Tsygankov
mesh_terms: D050788:Alanine Dehydrogenase; D000594:Amino Acid Oxidoreductases / Q000032:analysis; D000641:Ammonia / Q000378:metabolism*; D001432:Bacteriology / Q000295:instrumentation; D002952:Citric Acid Cycle; D015964:Gene Expression Regulation, Bacterial; D015971:Gene Expression Regulation, Enzymologic; D005969:Glutamate Dehydrogenase / Q000032:analysis; D005970:Glutamate Synthase / Q000032:analysis; D005974:Glutamate-Ammonia Ligase / Q000032:analysis; D016946:Gram-Negative Aerobic Rods and Cocci / Q000201:enzymology* / Q000254:growth & development; D007521:Isocitrate Dehydrogenase / Q000032:analysis; D000432:Methanol / Q000378:metabolism*; D009243:NAD / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D005723:gamma-Glutamyltransferase / Q000032:analysis
chemical_list: D009243:NAD; D009249:NADP; D000641:Ammonia; D007521:Isocitrate Dehydrogenase; D000594:Amino Acid Oxidoreductases; D050788:Alanine Dehydrogenase; D005970:Glutamate Synthase; D005969:Glutamate Dehydrogenase; D005723:gamma-Glutamyltransferase; D005974:Glutamate-Ammonia Ligase; D000432:Methanol