Regulation of expression of the GLN-τ gene of Phaseolus vulgaris L

In Phaseolus vulgaris glutamine synthetase (GS) is encoded by four expressed, nuclear genes (1), three of which (gln-α, gln-β and gln-τ) encode cytosolic GS polypeptides (α, β and τ) whereas the fourth gene (gln-δ) encodes a precursor to the plastid-located GS polypeptide (6). During nodulation the activity of GS increases about 8-fold in specific activity (2), presumably in order to assimilate the ammonium derived from dinitrogen fixation. Studies on isoenzymes of GS have shown that this increase is largely due to the production of τ-containing isoenzymes (2). The regulation of activity of the gln-τ gene therefore appears to be important tor the assimilation of dinitrogen-fixed ammonium.