The Effects of Gene Change on Tryptophan Desmolase Formation

Publisher Website
Google Scholar
Abstract
2 tryptophan-requiring mutations in Neurospora, td1 occurring in C-83, and td2 in S-1952, were each found to be due to a single gene, and, in allelism and reversion tests, behaved as alleles or closely linked genes. Both mutants were blocked in the conversion of indole to tryptophan and, by means of a sensitive test, were found to lack the tryptophan-catalyzing enzyme, tryptophan desmolase. Further tests with C-83 using N15 ring-labeled tryptophan indicated that C-83 did not synthesize a significant amt. of tryptophan. A microconidial stock of S-1952 which reverted to tryptophan independence was crossed with 2 wild type strains. Among the 3 types of resulting asci the most frequent type gave 6 tryptophan-independent (T+):2 tryptophan-dependent (T-) ascospores, suggesting that the reversion was due to a suppressor gene (su) at a separate locus. Tryptophan-independents from 4T+:4T-asci, when crossed to S-1952, again gave 3 types of asci. Growth curves of the various strains on minimal medium and on minimal + tryptophan showed that the mutant and wild strains carrying su grew poorly on minimal medium, but that addition of tryptophan stimulated the mutant-su strain but not the wild-su strain. A lag occurred in the growth of strains carrying su, which was not overcome by adding various supplements. Since su had no effect on crosses between wild-su and C-83, it was assumed to be specific for td2. Tryptophan desmolase activity was present in extracts of S-1952-su and of wild-su, but could not be detected in C-83-su. 2 genes, td2 and su, appeared to be required for tryptophan desmolase synthesis. The max. amt. of tryptophan desmolase in S-1952-su was approx. 5% of the max. amt. formed by wild type strains, and wild-su had a max. content about 1/3 that of wild type strains with-out su. Comparisons between tryptophan desmolase from a wild type strain and from S-1952-su with respect to several enzyme characteristics gave no major differences. In the light of these results the relationship between genes and enzymes is discussed.