SUMMARY: Methods have been developed which permit the measurement of -[U-C]isoleucine incorporation into glutamine synthetase so that the rate of enzyme synthesis can be measured during enzyme deactivation and reactivation. The results suggest that glutamine, a co-repressor of enzyme synthesis, affects enzyme translation. Pulse labelling experiments indicate that enzyme tetramers are not direct precursors of the active enzyme but instead represent a pool of ‘resting’ enzyme. ‘Fine’ control of activity is achieved by reversible aggregation of tetramers, with diminished biosynthetic activity, into active octamers. A number of negative and positive effectors can alter the equilibrium between the two enzyme forms. An earlier suggestion that the monomers produced by the irreversible dissociation of ‘tight’ enzyme tetramers cannot be re-utilized by the yeast has been confirmed. Proteolytic enzymes do not seem to be directly implicated in the destruction of the enzyme. A model is presented which incorporates all the available information on the control of glutamine biosynthesis in food yeast.


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