REGULATION OF EARLY REACTIONS IN THE BIOSYNTHESIS OF TRYPTOPHAN IN NEUROSPORA CRASSA

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Lester, Gabriel (Reed College, Portland, Ore.). Regulation of early reactions in the biosynthesis of tryptophan in Neurospora crassa. J. Bacteriol. 85:468–475. 1963.—The regulation of the biosynthesis of tryptophan was examined in Neurospora crassa, strain ylo-tryp-la, which accumulates anthranil compounds. The block in this strain appeared to be in the conversion of 1-(o-carboxyphenylamino)-1-deoxyribulose-5-phosphate to indole-3-glycerol phosphate, since the dephosphorylated form of the former compound, the anthranilic ribonucleoside, and the anthranilic acid were found. Cells cultured on levels of l-tryptophan greater than 0.1 μmole per ml were almost devoid of anthranilate-synthesizing activity, whereas cells cultured on low levels of tryptophan (e.g., 0.025 μmole/ml) could produce anthranilate at a rate of 125 mμmoles per mg (dry wt) per hr. A repressive effect was also caused by d-, 5-methyl-dl-, and 6-methyl-dl-tryptophan, but none of these compounds was as effective a repressor as l-tryptophan. Neither 4-methyl-dl-tryptophan, tryptamine, nor indole-3-acetic acid repressed the formation of anthranilate-synthesizing activity. Preformed activity was strongly inhibited by l-tryptophan, and to a lesser extent by 4-, 5-, and 6-methyl-dl-tryptophan; d-tryptophan, tryptamine, or indole-3-acetic acid did not inhibit preformed anthranilate-synthesizing activity. These results are indicative of the operation of repression and feedback-inhibition mechanisms early in the biosynthetic sequence leading to tryptophan. The relation of these results to those concerned with other aspects of tryptophan biosynthesis is discussed.

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