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

Department of Biology, Reed College, Portland, Oregon

ABSTRACT

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.