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J Bacteriol. 1979 March; 137(3): 1333-1339
Copyright © 1979, American Society for Microbiology. All Rights Reserved.

Regulation of Glutamate Dehydrogenases in nit-2 and am Mutants of Neurospora crassa

Anne H. Dantzig, Francis L. Wiegmann Jr. and Alvin Nason

¹ McCollum-Pratt Institute, The Johns Hopkins University, Baltimore, Maryland 21218

ABSTRACT

The regulation of the glutamate dehydrogenases was investigated in wild-type Neurospora crassa and two classes of mutants altered in the assimilation of inorganic nitrogen, as either nitrate or ammonium. In the wild-type strain, a high nutrient carbon concentration increased the activity of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-glutamate dehydrogenase and decreased the activity of reduced nicotinamide adenine dinucleotide (NADH)-glutamate dehydrogenase. A high nutrient nitrogen concentration had the opposite effect, increasing NADH-glutamate dehydrogenase and decreasing NADPH-glutamate dehydrogenase. The nit-2 mutants, defective in many nitrogen-utilizing enzymes and transport systems, exhibited low enzyme activities after growth on a high sucrose concentration: NADPH-glutamate dehydrogenase activity was reduced 4-fold on NH₄Cl medium, and NADH-glutamate dehydrogenase, 20-fold on urea medium. Unlike the other affected enzymes of nit-2, which are present only in basal levels, the NADH-glutamate dehydrogenase activity was found to be moderately enhanced when cells were grown on a low carbon concentration. This finding suggests that the control of this enzyme in nit-2 is hypersensitive to catabolite repression. The am mutants, which lack NADPH-glutamate dehydrogenase activity, possessed basal levels of NADH-glutamate dehydrogenase activity after growth on urea or L-aspartic acid media, like the wild-type strain, and possessed moderate levels (although three- to fourfold lower than the wild-type strain) on L-asparagine medium or L-aspartic acid medium containing NH₄Cl. These regulatory patterns are identical to those of the nit-2 mutants. Thus, the two classes of mutants exhibit a common defect in NADH-glutamate dehydrogenase regulation. Double mutants of nit-2 and am had lower NADH-glutamate dehydrogenase activities than either parent. A carbon metabolite is proposed to be the repressor of NADH-glutamate dehydrogenase in N. crassa.

Present address: Department of Human Genetics, Yale University School of Medicine, New Haven, CT 06510.

Deceased.

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