Figure 4.

GS regulation by glutamine and the relation between glutamine pool and internal ammonium concentration. (A) At low ambient ammonium concentrations, GS is the major ammonium assimilation enzyme, catalyzing the formation of glutamine from ammonium and glutamate (Reitzer, 2003). GS expression is repressed by glutamine (red line) through the NtrBC two-component signaling system (Hirschman et al, 1985; Reitzer and Magasanik, 1986; Jiang et al, 1998a, b) while the specific activity of GS is inhibited by adenylylation by GlnE whose activity is stimulated by glutamine (Kingdon et al, 1967; Wulff et al, 1967; Okano et al, 2010). (B) This negative feedback regulation of glutamine on GS imposes an obligatory relation between the internal NH₄⁺ concentration and the glutamine pool, described by Equation (5): see text and Supplementary Equations S18–S20 for details. As shown in the plot, this obligatory relation features a very weak dependence of the glutamine pool on the internal NH₄⁺ concentration, reflecting the homeostatic nature of negative feedback control by glutamine (Reitzer, 2003).