Regulation of ammonia assimilation in ammonia‐limited chemostat cultures of Escherichia coli ML 30: Evidence of bistability

Abstract

I n a preceding paper evidence of two stationary stable states (bistability) in the specific activity of glutamine synthetase (GS) in ammonia-limited steady-state cultures of Escherichia coli ML 30 at dilution rates (D) about 0.15 h-l was described (MULLER et aE. 1977). For better understanding of the regulation mechanisms leading to GS bistability chemostat experiments were performed over a wide range of dilution rates up t o D = 0.8 h-l. For each steady state the specific activities of GS and glutamate dehydrogenase (GDH) -the other key enzyme of the two NH, assimilation routes in E. coli -and in addition the remaining NH, concentration in the culture liquid were determined.Parallel to GS bistability two states of GDH activity andNH, concentration are found. The higher state of GS is connected with a lower GDH activity and NH, concentration. With rising D the GS activities decrease whereas GDH activities and NH, concentrations increase.Since no adenylation of the GS is detectable GS bistability seems to be regulated on the level of enzyme synthesis like GDH bistability. From t h e experimental findings a mathematical model is derived based on the bottle neck enzyme theory of growth. It describes the dependence between the specific growth rates on the one hand and the specific enzyme activities and NH, concentration on the other. It is shown t h a t the specific uptake rate of the limiting NH, and the specific growth rates, respectively, depend on the simultaneous action of two bottle neck enzymes which are connected by a regulative link.Chemostat cultures allow to investigate the enzyme regulation in microorganisms under steady-state conditions in dependence on the specific growth rate (p) or dilution rate (D), respectively. The situation is complicated by the possibility of bi-or multistability in the intracellular specific enzyme activities leading to different intraand extracellular metabolite concentrations (BERGTER 1968(BERGTER , 1969a 1977, 1979). The intracellular enzyme activity can be regulated on the epigenetic level, which may be indicated by experimental estimated enzyme activities, and/or on the metabolic level.More

Bistability in the glucose and energy metabolism of ammonia‐limited chemostat cultures of Escherichia coli ML 30

“…Because pyruvate bistability is connected with a bistability in the ammonia assimilation systems (BERGTER et , MULLER and BERGTER 1977, 1978, MULLER et al 2977, 1981 it was a t first assumed that a higher steady state ammonia concentration in state B activates the glucose uptake (G, -G) and pyruvate formation, leading to lower YBIQ. At the other side 2 and in this connection also Y x ,~ could be affected in the same direction by the stated differences in limiting NH, and/or by accumulation of glycogen in the cells, proved for E , coli chemostat cultures a t ammonia liniitation (HOLME 1958).…”