Mutations affecting phenol oxidase activity inDrosophila: quicksilver andtyrosinase-1

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Abstract
The complex enzyme phenol oxidase plays a major role in sclerotization and melanization of cuticle in insects. Production of active enzyme from the inactive proenzyme involves at least six protein components inDrosophila. We examine here the biochemical phenotype of two loci that affect phenol oxidase activity—quicksilver (qs; 1–39.5) andtyrosinase-1 (tyr-1; 2–54.5). Three mutations isolated by different procedures in three different laboratories are alleles at thequicksilver locus. The effects of these mutations have been monitored by means of enzyme assaysin vitro and in polyacrylamide gels and by measurement of catecholamine pool sizes. The activity of all three active enzyme components (A1, A2, and A3) is reduced inqs mutants. The activated enzyme of oneqs allele is thermolabile, while its activator is normal. Deletion and genetic mapping placetyr-1 nearpurple (pr; 2–54.5). Enzyme activity is reduced to 10% of normal but is not thermolabile and the activator is normal. The activity of all three A components is reduced. The diphenol oxidase activity in double mutant combinations shows that these mutations andDox-A2 (Pentzet al., 1986) affect this enzyme in different ways.