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Glutamine synthetase genes are regulated by ammonia provided externally or by symbiotic nitrogen fixation

Glutamine synthetase is the key enzyme in the assimilation by plants of reduced nitrogen provided from either the soil or fixed symbiotically in association with Rhizobium. We have isolated a number of cDNA clones for soybean glutamine synthetase (GS) from a nodule‐cDNA library, using RNA from polysomes immunoprecipitated by GS antibodies. Transcripts corresponding to two clones differing in their 3′ non‐translated sequences were present in both root and nodule tissue; however, the concentration in the nodules was several times higher. The relative concentrations of these sequences in both tissues is about 9:1. Availability of ammonium ions [provided as NH4NO3 or (NH4)2SO4] enhanced the expression of both sequences in root tissue within 2 h, reaching a level similar to that in nodules by 8 h, while KNO3 had no effect during this period. When nitrogen fixation was prevented by replacing nitrogen with argon in the root environment or when the nodules were formed by a Fix‐ mutant of Bradyrhizobium japonicum, the amounts of GS mRNA did not increase over that in roots. These experiments, together with the time course of increase in GS mRNA transcripts, suggest that the genes encoding cytosolic GS are directly induced by the available ammonia.