Electron microscopy of multiple forms of glutamine synthetase from bacteroids and the cytosol of yellow lupin root nodules

The quaternary structure of multiple forms of glutamine synthetase (EC 6.3.1.2) from bacteroids and cytosol of yellow lupin (Lupinus luteus L.) root nodules has been studied by means of electron microscopy and image processing. The multiple forms of glutamine synthetase were found to possess different quaternary structures, which are apparently, responsible for their different properties and, possibly, for different functions. It was shown that one of the two forms of cytosol glutamine synthetase (glutamine synthetase II) which is predominant in nitrogen fixation conditions has a cubic configuration with eight identical monomers arranged in two layers with point group symmetry 422 (D₄). Two of the three bacteroid glutamine synthetase forms, glutamine synthetase II and glutamine synthetase III, possess different quarternary structures. Thus, bacteroid glutamine synthetase II was found to consist of eight identical monomers arranged with point group symmetry 422 (D₄) at the vertices of two squares which, unlike cytosol glutamine synthetase II are twisted about the 4-fold axis relative to each other. The quarternary structure of bacteroid glutamine synthetase II is typical for glutamine synthetases of higher plants and may indicate its plant origin. As with bacterial glutamine synthetase, bacteroid glutamine synthetase III consists of twelve identical monomers arranged with point group symmetry 622 (D₆) at the vertices of two rectilinear hexagons. Computer averaging of bacteroid glutamine synthetase III images revealed a two-domain structure of its monomers.