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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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Related in: MedlinePlus

Steady maintenance of internal ammonium and the abrupt activation of AmtB. (A) The deduced internal NH4+ concentrations of the ΔamtB strain (EQ130, open purple circles) and wild type (EQ66, solid purple circles) grown in glycerol with varying NH4+ concentrations; see Equations (1) and (2), and the text. The gray region (defined in Figure 1E) indicates the conditions in which the ΔamtB strain exhibits reduced growth. (B) The dependences of the GS (left axis) and amtB (right axis) promoter activities of the ΔamtB strain on the deduced internal NH4+ concentrations. (C) The growth rate of the ΔamtB strain and wild type on the deduced internal NH4+ concentrations. (D) The Vmax of GS (VGS, brown), the GS expression level (PGS, red) and the specific activity of GS (kGS, cyan) are plotted relative to their values in ammonium-replete conditions. Here, VGS is obtained from Equation (3), and kGS is obtained from the ratio of VGS and PGS. The lines of respective colors are fit using Hill functions (Supplementary Equation S18); see Supplementary Table 4 for the kinetic parameters of the fit. (E) The deduced ammonium transport flux through AmtB (green circles) and the NH3 diffusion flux (blue circles); see Equations (1), (2) and (4). Black crosses represent the net nitrogen influx (the sum of the two fluxes), which the cells utilize for biomass synthesis. Note that the maintenance of the net flux is accompanied by strong increases in ammonium transport by AmtB, and NH3 leakage through passive diffusion. (F) A sharp increase in the ammonium transport flux through AmtB (green circles) when plotted against the deduced internal NH4+ concentrations.
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f2: Steady maintenance of internal ammonium and the abrupt activation of AmtB. (A) The deduced internal NH4+ concentrations of the ΔamtB strain (EQ130, open purple circles) and wild type (EQ66, solid purple circles) grown in glycerol with varying NH4+ concentrations; see Equations (1) and (2), and the text. The gray region (defined in Figure 1E) indicates the conditions in which the ΔamtB strain exhibits reduced growth. (B) The dependences of the GS (left axis) and amtB (right axis) promoter activities of the ΔamtB strain on the deduced internal NH4+ concentrations. (C) The growth rate of the ΔamtB strain and wild type on the deduced internal NH4+ concentrations. (D) The Vmax of GS (VGS, brown), the GS expression level (PGS, red) and the specific activity of GS (kGS, cyan) are plotted relative to their values in ammonium-replete conditions. Here, VGS is obtained from Equation (3), and kGS is obtained from the ratio of VGS and PGS. The lines of respective colors are fit using Hill functions (Supplementary Equation S18); see Supplementary Table 4 for the kinetic parameters of the fit. (E) The deduced ammonium transport flux through AmtB (green circles) and the NH3 diffusion flux (blue circles); see Equations (1), (2) and (4). Black crosses represent the net nitrogen influx (the sum of the two fluxes), which the cells utilize for biomass synthesis. Note that the maintenance of the net flux is accompanied by strong increases in ammonium transport by AmtB, and NH3 leakage through passive diffusion. (F) A sharp increase in the ammonium transport flux through AmtB (green circles) when plotted against the deduced internal NH4+ concentrations.

Mentions: The internal NH4+ concentrations ([NH4+]ext) of ΔamtB cells, deduced from Equations (1) and (2) with Jdiffusion=Jbiomass using the measured growth rate λΔamtB for each external NH4+ concentration (open circles in Figure 1E), are plotted for various external NH4+ concentrations as shown in Figure 2A (open purple circles); see Supplementary Equations S8–S11. The linear relation is expected due to the passive diffusion of NH3 in ΔamtB cells. Here and below, the gray region, adopted from that in Figure 1E, indicates the conditions in which the ΔamtB strain exhibits reduced growth.


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)

Steady maintenance of internal ammonium and the abrupt activation of AmtB. (A) The deduced internal NH4+ concentrations of the ΔamtB strain (EQ130, open purple circles) and wild type (EQ66, solid purple circles) grown in glycerol with varying NH4+ concentrations; see Equations (1) and (2), and the text. The gray region (defined in Figure 1E) indicates the conditions in which the ΔamtB strain exhibits reduced growth. (B) The dependences of the GS (left axis) and amtB (right axis) promoter activities of the ΔamtB strain on the deduced internal NH4+ concentrations. (C) The growth rate of the ΔamtB strain and wild type on the deduced internal NH4+ concentrations. (D) The Vmax of GS (VGS, brown), the GS expression level (PGS, red) and the specific activity of GS (kGS, cyan) are plotted relative to their values in ammonium-replete conditions. Here, VGS is obtained from Equation (3), and kGS is obtained from the ratio of VGS and PGS. The lines of respective colors are fit using Hill functions (Supplementary Equation S18); see Supplementary Table 4 for the kinetic parameters of the fit. (E) The deduced ammonium transport flux through AmtB (green circles) and the NH3 diffusion flux (blue circles); see Equations (1), (2) and (4). Black crosses represent the net nitrogen influx (the sum of the two fluxes), which the cells utilize for biomass synthesis. Note that the maintenance of the net flux is accompanied by strong increases in ammonium transport by AmtB, and NH3 leakage through passive diffusion. (F) A sharp increase in the ammonium transport flux through AmtB (green circles) when plotted against the deduced internal NH4+ concentrations.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Steady maintenance of internal ammonium and the abrupt activation of AmtB. (A) The deduced internal NH4+ concentrations of the ΔamtB strain (EQ130, open purple circles) and wild type (EQ66, solid purple circles) grown in glycerol with varying NH4+ concentrations; see Equations (1) and (2), and the text. The gray region (defined in Figure 1E) indicates the conditions in which the ΔamtB strain exhibits reduced growth. (B) The dependences of the GS (left axis) and amtB (right axis) promoter activities of the ΔamtB strain on the deduced internal NH4+ concentrations. (C) The growth rate of the ΔamtB strain and wild type on the deduced internal NH4+ concentrations. (D) The Vmax of GS (VGS, brown), the GS expression level (PGS, red) and the specific activity of GS (kGS, cyan) are plotted relative to their values in ammonium-replete conditions. Here, VGS is obtained from Equation (3), and kGS is obtained from the ratio of VGS and PGS. The lines of respective colors are fit using Hill functions (Supplementary Equation S18); see Supplementary Table 4 for the kinetic parameters of the fit. (E) The deduced ammonium transport flux through AmtB (green circles) and the NH3 diffusion flux (blue circles); see Equations (1), (2) and (4). Black crosses represent the net nitrogen influx (the sum of the two fluxes), which the cells utilize for biomass synthesis. Note that the maintenance of the net flux is accompanied by strong increases in ammonium transport by AmtB, and NH3 leakage through passive diffusion. (F) A sharp increase in the ammonium transport flux through AmtB (green circles) when plotted against the deduced internal NH4+ concentrations.
Mentions: The internal NH4+ concentrations ([NH4+]ext) of ΔamtB cells, deduced from Equations (1) and (2) with Jdiffusion=Jbiomass using the measured growth rate λΔamtB for each external NH4+ concentration (open circles in Figure 1E), are plotted for various external NH4+ concentrations as shown in Figure 2A (open purple circles); see Supplementary Equations S8–S11. The linear relation is expected due to the passive diffusion of NH3 in ΔamtB cells. Here and below, the gray region, adopted from that in Figure 1E, indicates the conditions in which the ΔamtB strain exhibits reduced growth.

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