Distinct Properties of Two Glutamine Synthetase Isoforms in Soybean Root Nodules

Glutamine synthetase (GS) in nodule cytosol plays a major role in the assimilation of the ammonium produced by biological nitrogen fixation. To characterize the GS protein family in Glycine max root nodules, the catalytical properties of 2 GS1 isoenzymes (GS1 beta 1 and GS1 gamma 1) were compared in this study. Although, GmGS1 beta 1 and GmGS1 gamma 1 have very similar kinetic characteristics, they also exhibit distinct enzymatic properties in terms of thermal stability and the transferase to synthetase activity (GSt/GSs) ratios. The results demonstrated that GmGS1 gamma 1, which displayed lower thermal stability and GSt/GSs ratios than GmGS1 beta 1, might be considered as superior isoform to participate in the efficient assimilation of ammonia only when it is needed. Also, it is proposed that the difference of enzymatic properties between isoforms contribute to their differential roles in ammonia assimilation under variable internal and external environments.