The effect of the carbon and nitrogen source on the regulation of glutamine synthetase (GS) has been studied by omitting one or both from mycelium which had been grown in sucrose/ glutamate medium, a condition in which GS is fully induced in its octameric form. The activity and antigen concentration of GS decreased when mycelium was deprived of the carbon source. Even more enzyme was degraded when glutamate was replaced by glutamine and an oligomeric form of GS lower than the octamer was also observed. When the carbon source was restored, some of the enzyme was resynthesized during incubation with glutamate but not with glutamine. In both cases the final predominant oligomeric form of GS was the tetramer. When the carbon and nitrogen sources were both omitted from the medium, degradation of GS again occurred and the restoration of both sources did not result in an increase in GS: no change in the oligomeric state of GS was observed during these shifts. The deprivation of the nitrogen source resulted in the replacement of the octamer by the highly active tetrameric form of GS. These changes were not reversed when either the carbon or the nitrogen source was again supplied.


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