Incomplete Genetic Blocks in Biochemical Mutants of Neurospora

Abstract

Tracer atoms and measurement of the accumulation of specific intermediates by double mutants were used to determine the completeness of genetic blocks. With N15 -labeled anthranilic acid, a tryptophanless mutant, strain 39401, was found to convert anthranilic acid to tryptophan when grown on niacin alone. To further test this behavior, double mutants were formed by combining each of 4 tryptophanless strains ( C-83, 10575, 75001 and 7655 ) with strain 3416, which requires niacin for growth and accumulates quinolinic acid. When double mutant C-83-3416 was grown on Nl5-labeled tryptophan and unlabeled niacin, the isolated tryptophan and quinolinic acid, which had formed, had the same N15 content as the fed tryptophan, indicating that the C-83 strain caused a complete block to tryptophan formation. With the other 3 double mutants substantial portions of the isolated tryptophan and quinolinic acid were unlabeled and must have been derived from constituents of the minimal medium, indicating that the genetic block was incomplete. There was no evidence of reversion of mutant loci in any of the expts. Since exchange of Nl5 in the conversion of anthranilic acid to tryptophan had not been completely ruled out, tests were run to see whether strains could form tryptophan de novo. When the double mutants 10575-3416 and 75001-3416 were grown on different levels of indole, the quinolinic acid accumulation was not proportional to indole concn. Thus the strains were capable of forming tryptophan under growing conditions. In connection with the possibility of alternative biochemical pathways, tests were made to detn. whether niacinless mutants could form a required substance during growth. Strains 31881 and 4540, which cause genetic blocks in niacin formation, were each combined with 3416, and the double mutants were grown on niacin and tested for quinolinic acid accumulation. The 4540 block was complete, but the 31881 block was incomplete as evi-denced by the formation of quinolinic acid. The ability to form compounds such as tryptophan or niacin was reflected in growth, as shown by the reactions of strains C-83 and 10575 when grown on different levels of tryptophan. Strain C-83, which cannot form tryptophan, reached diff. final levels of growth depending on the concn. of tryptophan, whereas 10575 reached about the same level of growth regardless of tryptophan concn. The phenomenon whereby a mutant strain, which cannot grow in the absence of a given substance, may be capable of forming this substance under growing conditions is termed"leakage" by the authors.