SUMMARY: Two strains of , which require α-amino nitrogen for growth as a result of a single gene mutation (), lack glutamic dehydrogenase. Extracts of mutant mycelium contained no detectable enzyme, although as little as 0.2% of the amount normally present in the wild type should have been detectable in some experiments. Mutant extracts contained no detectable enzyme inhibitor. A balanced heterocaryon containing nuclei and nuclei carrying an unrelated mutation produced less enzyme than did wild type mycelium, but there was no indication that the nuclei were suppressing the activity of the non- nuclei in promoting enzyme production. The production of the enzyme by the wild type was not strikingly dependent on conditions of growth although the addition of glutamate to the medium seemed somewhat to depress the enzyme concentration. This effect was more marked when glutamate was supplied as a sole nitrogen source. The mutation had little or no effect on the concentrations of two other dehydrogenases or of several transaminases, all related to glutamic dehydrogenase in their substrate specificities. The possible significance of these observations is discussed.


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