Abstract
Cells of wild-type strains of Neurospora produce a TPN-dependent and a DPN-dependent glutamic dehydrogenase in a basal medium containing ammonium nitrate. In the presence of exogenous glutamate during growth, the specific activity of the TPN-specific enzyme is considerably decreased, and this is accompanied by a corresponding increase in the activity of the DPN-specific enzyme. This effect is observed only when the cells are growing in the presence of glutamate and ammonia together; individually these substances show no effect. Glutamate can be replaced by aspartate, asparagine, alanine, arginine, or ornithine. In auxotrophs requiring arginine, ornithine, or citrulline, however, exogenous glutamate has no effect on the activity of the glutamic dehydrogenases. From this it is inferred that a substance produced from glutamate and ammonia through the Krebs-Henseleit urea cycle is the actual cause of the effect shown on the two enzymes. This compound is probably urea, or an “active” derivative of urea. The mechanism of this “urea effect” is discussed in the light of contemporary understanding of the regulation of enzyme synthesis in microorganisms.
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