Figure 7.

Model for the role of the three NadE in A. brasilense. A, under nitrogen-fixing conditions nitrogenase feeds ammonium into GS, and intracellular levels of glutamine are low (∼80 μm). NadE1^Gln operates at V_max/2 and promptly responds to small fluctuations in glutamine availability within this range (see the blue bar with Michaelis–Menten curves in D). NadE2^Gln and NadE^NH3 activities are negligible leading to slow production of NAD⁺. B, upon an ammonium shock of NH₄⁺ 200 μm, the intracellular glutamine rises to ∼800 μm, leading to total inhibition of nitrogenase by ADP-ribosylation and nearly full inhibition of GS by adenylylation. NadE1^Gln operates at V_max, whereas NadE2^Gln operates at V_max/2 and promptly responds to small fluctuations in glutamine availability within this range (see the green bar with Michaelis–Menten curves in D). NAD⁺ production is elevated. C, when external levels of ammonium are high, the high intracellular glutamine will lead to NadE1^Gln and NadE2^Gln to operate at V_max using glutamine as substrate. Furthermore, because GS is fully inactive due adenylylation, free intracellular ammonium could be directly assimilated into NAD⁺ by NadE2^Gln and presumably also by NadE^NH3 bypassing GS activity. The concert action of all three isoforms of NadE is likely to further enhance NAD⁺ production. D, velocities of the different A. brasilense NadE isoforms accordingly to the intracellular glutamine levels.