ORGAN-SPECIFIC CHANGES IN THE ACTIVITY AND SUBUNIT COMPOSITION OF GLUTAMINE-SYNTHETASE ISOFORMS OF BARLEY (HORDEUM-VULGARE L) AFTER GROWTH ON DIFFERENT LEVELS OF NH4+

One cytosolic glutamine synthetase (GS, EC 6.3.1.2) isoform (GS 1a) wa s active in the germinating seeds of barley (Hordeum vulgare L.). A se cond cytosolic GS isoform (GS 1b) was separated from the leaves as wel l as the roots of 10-d-old seedlings. The chloroplastic isoform (GS 2) was present and active only in the leaves. The three GS isoforms were active in N-supplied (NH4+ or NO3-) as well as in N-free-grown seedli ngs. This indicates (i) that a supply of nitrogen to the germinating s eeds was not necessary for the induction of the GS isoforms and (ii) t hat no nitrogen-specific isoforms appeared during growth of seedlings with different nitrogen sources. The activity of GS, however, depended on the seedlings' nitrogen source: the specific activity was much hig her in the leaves and much lower in the roots of NH4+-grown barley tha n in the respective organs of NO3--fed or N-free-grown plants. With in creasing concentrations of NH4+ (supplied hydroponically during growth ), the specific activity of GS 1b increased in the leaves, but decreas ed in the roots. The activity of GS 2 (leaf) also increased with incre asing NH4+ supply, whereas GS 1a activity (leaf and root) was not affe cted. The changes in the activities of GS 1b and GS 2 were correlated with changes in the subunit compositions of the active holoenzymes: gr owth at increased levels of external NH4+ resulted in an increased abu ndance of one of the four GS 2 subunits, and of two of the five GS 1b subunits in the leaves. In the roots, however, the abundance of these two GS 1b subunits was decreased under the same growth conditions, ind icating an organ-specific difference either in the expression of the g enes coding for the respective GS 1b subunits or in the assembly of th e GS 1b holoenzymes. Furthermore, growth at different levels of NH4+ r esulted in changes in the substrate affinities of the isoforms GS 1b ( root and leaf) and GS 2 (leaf), presumably due to the changes in the s ubunit compositions of the active holoenzymes.