A Diphenol Oxidase Gene Is Part of a Cluster of Genes Involved in Catecholamine Metabolism and Sclerotization in Drosophila. I. Identification of the Biochemical Defect in Dox-A2 [l(2)37Bf] Mutants

Phenol oxidase, a complex enzyme, plays a major role in the processes of sclerotization and melanization of cuticle in insects. Several loci have been reported to affect levels of phenol oxidase activity, but to date only one structural locus has been identified Dox-3F (2-53.1+)]. Recently isolated Dox-A2 mutations (2-53.9) are recessive, early larval lethals, which as heterozygotes reduce phenol oxidase activity. A homozygous mutant escaper had weak, completely unpigmented cuticle and unpigmented bristles. Enzyme assays show that Dox-A2 heterozygotes have diphenol oxidase activity reduced to 47-79% of wild type, whereas monophenol oxidase activity, at 94-106% of wild type, is normal. Elevated pool sizes of the diphenol oxidase substrates DOPA, dopamine, and N-acetyldopamine are observed in the mutant, confirming the enzyme assay results. Separation of the three phenol oxidase A component activities on polyacrylamide gels shows that Dox-A2 mutations reduce the activity of only the A2 component. Dox-A2 may identify a structural locus for the A2 component of the diphenol oxidase enzyme system. The Dox-A2 locus is one of 18 loci in the dopa decarboxylase, Df (2L)TW130 region of the second chromosome, at least 14 of which affect the formation, melanization or sclerotization of cuticle in some way. These loci form an apparent cluster of functionally related genes.