Yeast glutamine synthetase was purified and shown to be an octameric globular protein (= 15.4, = 1·29, mol. wt= 390000). It consists of two weakly-bound half molecules (= 8·7, = 1·35, mol. wt= 180500) and relatively harsh treatment is required to dissociate these octamers into component monomers (= 3·8). Deactivation of the enzyme may include changes in the conformation of the native enzyme with its separation into two ‘tight’ tetramers, followed by their dissociation into component monomers and dimers. In the presence of Mg and glutamate, monomers re-aggregate to oligomers which, although having transferase activity, are devoid of synthetase activity. The relevance of these observations in relation to control of glutamine synthetase activity in yeast is discussed.


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