REGULATION OF AMMONIA ASSIMILATION IN AN OBLIGATE METHYLOTROPH METHYLOBACILLUS-FLAGELLATUM UNDER STEADY-STATE AND TRANSIENT GROWTH-CONDITIONS

The obligate methylotroph Methylobacillus flagellatum was grown in the presence of different ammonium concentrations and the regulation of t he enzymes associated with ammonium assimilation was investigated in s teady-state and transient growth regimes. As the medium changed from C -limitation to dual C/N- and finally to N-limitation, the culture pass ed through three definite growth phases. The NADP(+)-dependent glutama te dehydrogenase (GDH) was present under ammonium limitation of the cu lture growth (at 2 mmol l(-1) of ammonium in the growth medium) and in creased in response to an increase in nitrogen availability. Glutamine synthetase (GS) and glutamate synthase (GOGAT) activities were neglig ible during C- and C/N-limitation. In N-limited cells the GOGAT activi ty 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) activiti es were up-regulated as dilution rate increased, but in the N-limited culture the NAD-ICDH activity was up-regulated whereas NADP-ICDH one w as down-regulated. Pulse additions of ammonium and methanol demonstrat ed the coordinate regulation of the GDH and ICDHs activities. When pul ses were added to the C/N-limited cultures, there was an immediate uti lization 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 respo nse, after which the substrates were utilized at rates close to the on es shown by the C/N-limited culture after the analogous pulse.