The Molecular Biology and Biochemistry of Plant Glutamine Synthetase from Root Nodules of Phaseolus vulgaris L. and other Legumes

Ammonia produced by the bacterial fixation of dinitrogen in legume root nodules is assimilated by the plant glutamine synthetase (GS). Biochemical studies have shown that plant nodule GS consists of one or more major cytosolic and a minor plastidic GS polypeptide which assemble to form distinct octameric GS isoenzymes. Recombinant DNA techniques have shown that these different GS polypeptides are all encoded by separate nuclear genes which are expressed differentially in nodules, roots and leaves. In Phaseolus vulgaris L. GS activity increases several fold during nodulation and this is accounted for by the induction of a GS gene expressed specifically in nodules which produces a cytosolic GS polypeptide and isoenzyme. Three other GS genes, encoding two cytosolic and a plastidic GS, are also expressed in these nodules but at much lower levels. Other legumes, for example Pisum sativum L. do not appear to possess a nodule-specifically expressed GS gene but mediate an increase in nodule GS activity by expressing at a higher level, GS genes which are also transcribed in other organs. The regulation of both the expression of the GS genes and of the assembly and activity of nodule GS isoenzymes are also described in this review.