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Need-based activation of ammonium uptake in Escherichia coli.

Kim M, Zhang Z, Okano H, Yan D, Groisman A, Hwa T - Mol. Syst. Biol. (2012)

Bottom Line: We find that as the ambient ammonium concentration is reduced, E. coli cells first maximize their ability to assimilate the gaseous NH3 diffusing into the cytoplasm and then abruptly activate ammonium transport.Quantitative modeling of known interactions reveals an integral feedback mechanism by which this need-based uptake strategy is implemented.This novel strategy ensures that the expensive cost of upholding the internal ammonium concentration against back diffusion is kept at a minimum.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of California at San Diego, La Jolla, CA, USA.

ABSTRACT
The efficient sequestration of nutrients is vital for the growth and survival of microorganisms. Some nutrients, such as CO2 and NH3, are readily diffusible across the cell membrane. The large membrane permeability of these nutrients obviates the need of transporters when the ambient level is high. When the ambient level is low, however, maintaining a high intracellular nutrient level against passive back diffusion is both challenging and costly. Here, we study the delicate management of ammonium (NH4+/NH3) sequestration by E. coli cells using microfluidic chemostats. We find that as the ambient ammonium concentration is reduced, E. coli cells first maximize their ability to assimilate the gaseous NH3 diffusing into the cytoplasm and then abruptly activate ammonium transport. The onset of transport varies under different growth conditions, but always occurring just as needed to maintain growth. Quantitative modeling of known interactions reveals an integral feedback mechanism by which this need-based uptake strategy is implemented. This novel strategy ensures that the expensive cost of upholding the internal ammonium concentration against back diffusion is kept at a minimum.

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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).
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f4: 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).

Mentions: The delicate control of AmtB activity and its coordination with cellular nitrogen demand require intricate signaling system(s). Glutamine has long been established as a signal of the internal nitrogen status (Reitzer, 2003), serving as the major effector of GS expression (Hirschman et al, 1985; Reitzer and Magasanik, 1986; Jiang et al, 1998a, b) and activity (Kingdon et al, 1967; Wulff et al, 1967; Okano et al, 2010) (Figure 4A). It is therefore a possible candidate for controlling AmtB activity, via for example, the known effect of glutamine on GlnK that inhibits AmtB (Coutts et al, 2002; Javelle et al, 2004). The intracellular glutamine concentration unfortunately cannot be directly measured for cells grown under the very low ammonium conditions provided by the microfluidic chambers, due to the small number of cells in the chambers. However, because GS is the sole assimilator of ammonium at low ambient NH4+ concentrations (Reitzer, 2003), the negative feedback regulation of glutamine on GS imposes an obligatory relation between the internal NH4+ concentration and the glutamine pool which can be deduced from two relations: (i) the empirically obtained relation between the relative PGS-mCherry promoter activity and the internal NH4+ concentration, PGS([NH4+]int) (red circles and line in Figure 2D) and (ii) the mechanism regulating GS expression, controlled primarily by the glutamine concentration [Gln] under nitrogen limitation (Hirschman et al, 1985; Reitzer and Magasanik, 1986; Jiang et al, 1998a, b; Reitzer, 2003).


Need-based activation of ammonium uptake in Escherichia coli.

Kim M, Zhang Z, Okano H, Yan D, Groisman A, Hwa T - Mol. Syst. Biol. (2012)

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).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472687&req=5

f4: 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).
Mentions: The delicate control of AmtB activity and its coordination with cellular nitrogen demand require intricate signaling system(s). Glutamine has long been established as a signal of the internal nitrogen status (Reitzer, 2003), serving as the major effector of GS expression (Hirschman et al, 1985; Reitzer and Magasanik, 1986; Jiang et al, 1998a, b) and activity (Kingdon et al, 1967; Wulff et al, 1967; Okano et al, 2010) (Figure 4A). It is therefore a possible candidate for controlling AmtB activity, via for example, the known effect of glutamine on GlnK that inhibits AmtB (Coutts et al, 2002; Javelle et al, 2004). The intracellular glutamine concentration unfortunately cannot be directly measured for cells grown under the very low ammonium conditions provided by the microfluidic chambers, due to the small number of cells in the chambers. However, because GS is the sole assimilator of ammonium at low ambient NH4+ concentrations (Reitzer, 2003), the negative feedback regulation of glutamine on GS imposes an obligatory relation between the internal NH4+ concentration and the glutamine pool which can be deduced from two relations: (i) the empirically obtained relation between the relative PGS-mCherry promoter activity and the internal NH4+ concentration, PGS([NH4+]int) (red circles and line in Figure 2D) and (ii) the mechanism regulating GS expression, controlled primarily by the glutamine concentration [Gln] under nitrogen limitation (Hirschman et al, 1985; Reitzer and Magasanik, 1986; Jiang et al, 1998a, b; Reitzer, 2003).

Bottom Line: We find that as the ambient ammonium concentration is reduced, E. coli cells first maximize their ability to assimilate the gaseous NH3 diffusing into the cytoplasm and then abruptly activate ammonium transport.Quantitative modeling of known interactions reveals an integral feedback mechanism by which this need-based uptake strategy is implemented.This novel strategy ensures that the expensive cost of upholding the internal ammonium concentration against back diffusion is kept at a minimum.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of California at San Diego, La Jolla, CA, USA.

ABSTRACT
The efficient sequestration of nutrients is vital for the growth and survival of microorganisms. Some nutrients, such as CO2 and NH3, are readily diffusible across the cell membrane. The large membrane permeability of these nutrients obviates the need of transporters when the ambient level is high. When the ambient level is low, however, maintaining a high intracellular nutrient level against passive back diffusion is both challenging and costly. Here, we study the delicate management of ammonium (NH4+/NH3) sequestration by E. coli cells using microfluidic chemostats. We find that as the ambient ammonium concentration is reduced, E. coli cells first maximize their ability to assimilate the gaseous NH3 diffusing into the cytoplasm and then abruptly activate ammonium transport. The onset of transport varies under different growth conditions, but always occurring just as needed to maintain growth. Quantitative modeling of known interactions reveals an integral feedback mechanism by which this need-based uptake strategy is implemented. This novel strategy ensures that the expensive cost of upholding the internal ammonium concentration against back diffusion is kept at a minimum.

Show MeSH
Related in: MedlinePlus