SOME ASPECTS OF TRYPTOPHAN SYNTHETASE FORMATION IN NEUROSPORA CRASSA

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Abstract
The formation of tryptophan synthetase was examined in germinated conidia of a tryptophan-independent strain, 74A, and a tryptophan-dependent strain, 10575A, blocked after anthranilic acid in the biosynthesis of tryptophan. L-tryptophan had no effect, but 5-methyl-, 6-methyltrypto-phan, and indole-3-acetic acid caused a 1.5-2.5 fold stimulation of tryptophan-synthetase formation. In strain 10575A the level of tryptophan synthetase was inversely related to the concentration of L-tryptophan in the growth medium, with a 3-fold difference between the lowest and highest activities obtained. Of several other indolyl compounds examined only 6-methyltryptophan and indole depressed tryptophan-synthetase formation. None of the compounds which were stimulatory in strain 74A had a similar effect in strain 10575A. In contrast to the relatively narrow range of variation in tryptophan-synthetase formation, the formation of anthranilio-acid-synthesizing activity in strain 10575A was almost completely inhibited by L-tryptophan, and preformed activity was markedly inhibited by L-tryptophan. These observations suggested the operation of repression and negative-feedback control of tryptophan biosynthesis in N. crassa; it also appeared that these regulatory mechanisms were most pronounced at points in the bio-synthetic sequence preceding the terminal, tryptophan-synthetase reaction.