The Nature of the Lesion in the Succinate-requiring Mutants of Neurospora crassa: Interaction between Carbohydrate and Nitrogen Metabolism

Abstract

SUMMARY: Mutants of Neurospora crassa requiring dicarboxylic acids for an immediate growth response (suc and at suc) oxidize acetate, and are inhibited by fluoroacetate with consequent citric acid accumulation to approximately the same extent as the wild-type. The concentration of nitrogen (as ammonium and nitrate salts) present in the conventional growth medium is inhibitory to the growth of these mutants and leads to an accumulation of acetylmethylcarbinol, pyruvic acid and α-ketoisovaleric acid. This inhibition is reduced and growth is stimulated by the addition of dicarboxylic acids or by diminution of the nitrogen present in ‘minimal’ medium. The addition of nitrogen salts to suc mutants probably diverts dicarboxylic acids (already in short supply) from the catalysis of the oxidation of C2 fragments via the tricarboxylic acid cycle to other reactions. This effect of nitrogen salts upsets the already precarious dicarboxylic acid balance of the suc mutants leading to a growth requirement and to the accumulation of intermediates.