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Analysis of Pyrimidine Catabolism in Drosophila melanogaster Using Epistatic Interactions With Mutations of Pyrimidine Biosynthesis and β-Alanine Metabolism

The biochemical pathway for pyrimidine catabolism links the pathways for pyrimidine biosynthesis and salvage with β-alanine metabolism, providing an array of epistatic interactions with which to analyze mutations of these pathways. Loss-of-function mutations have been identified and characterized for each of the enzymes for pyrimidine catabolism: dihydropyrimidine dehydrogenase (DPD), su(r) mutants; dihydropyrimidinase (DHP), CRMP mutants; β-alanine synthase (βAS), pyd3 mutants. For all three genes, mutants are viable and fertile and manifest no obvious phenotypes, aside from a variety of epistatic interactions. Mutations of all three genes disrupt suppression by the rudimentary gain-of-function mutation (rSu(b)) of the dark cuticle phenotype of black mutants in which β-alanine pools are diminished; these results confirm that pyrimidines are the major source of β-alanine in cuticle pigmentation. The truncated wing phenotype of rudimentary mutants is suppressed completely by su(r) mutations and partially by CRMP mutations; however, no suppression is exhibited by pyd3 mutations. Similarly, su(r) mutants are hypersensitive to dietary 5-fluorouracil, CRMP mutants are less sensitive, and pyd3 mutants exhibit wild-type sensitivity. These results are discussed in the context of similar consequences of 5-fluoropyrimidine toxicity and pyrimidine catabolism mutations in humans.