4-Methyl-DL-tryptophan

The in vivo regulation of intermediate reactions in the pathway of tryptophan synthesis in Neurospora crassa was examined in a double mutant (tr-2, tr-3) which lacks the functions of the first and last enzymes in the pathway from chorismic acid to tryptophan. The double mutant can convert anthranilic acid to indole and indole-3-glycerol, and the production of these indolyl compounds by germinated conidia was used to estimate the activity of the intermediate enzymes in the pathway. Indole-synthesizing activity was maximal in germinated conidia obtained from cultures in which the levels of l-tryptophan were growth-limiting; the formation of this activity was markedly repressed when the levels of l-tryptophan exceeded those required for maximal growth. d-, 5-methyl-dl-, and 6-methyl-dl-tryptophan were less effective than l-tryptophan, and 4-methyl-dl-tryptophan, tryptamine, and indole-3-acetic acid were ineffective in repressing the formation of indole-synthesizing activity; anthranilic acid stimulated the formation of indole-synthesizing activity. Preformed indole-synthesizing activity was strongly and specifically inhibited by low levels of l-tryptophan; several related compounds were ineffective as inhibitors. These results suggest that, in addition to repression, an end product feedback inhibition mechanism is operative on an intermediate enzyme(s) in tryptophan biosynthesis. The relation of these results to other in vivo and in vitro studies and to general aspects of the regulation of tryptophan biosynthesis in N. crassa are discussed.

A colorless strain of Xanthomonas pruni was isolated which is capable of converting tryptophan to nicotinamide adenine dinucleotide (NAD). The enzymes responsible for the conversion of tryptophan to quinolinic acid were shown to be present. Nicotinic acid-requiring mutants were isolated, and it was found that the growth of these mutants can be supported by various intermediates on the pathway from tryptophan to NAD. The first three enzymes on this pathway are induced coordinately by l-tryptophan. Gratuitous inducers of these enzymes include d-tryptophan, alpha-methyl-dl-tryptophan, and 4-methyl-dl-tryptophan; formyl-l-kynurenine and l-kynurenine were not effective as inducers. These data suggest that at least the first three enzymes in the pathway from tryptophan to NAD are under common regulatory control.

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 mumole per ml were almost devoid of anthranilate-synthesizing activity, whereas cells cultured on low levels of tryptophan (e.g., 0.025 mumole/ml) could produce anthranilate at a rate of 125 mmumoles 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.