Pyrimidine biosynthesis in the dumpy mutants of Drosophila melanogaster.

The status of de novo pyrimidine synthesis in the dp mutant of Drosophila melanogaster was examined by measuring the activity of the rate-limiting orotate phosphribosyl transferase (OPRT) enzyme. Activity is significantly elevated in late third instar larvae of 5 different dp mutant strains. A more detailed analysis of a dpovc allele has shown that this elevation arises at about mid-larval life and persists until pupation. A low nucleotide diet causes a depression in OPRT activity in dpovc larvae which can be reversed by dietary supplementation of uracil. However, neither the low nucleotide diet nor uracil supplementation results in a change in the expressivity of the dp mutant phenotypes. Changes in expressivity are produced by 6-azauracil and by elevated temperature although, in those cases, the effect on OPRT activity is minimal. The significance of the observations is discussed in relation to the role of pyrimidine biosynthesis in dp expressivity and chitin synthesis.