The subcellular distribution of the first three enzymes in the ommochrome eye pigment pathway were studied by means of differential centrifugation. Tryptophan pyrrolase and kynurenine formamidasc are found in the soluble fraction of thc homogcnate. A relatively small amount of tryptophan pyrrolase, 10 to 15% of thc total, is sedimentable. The head region of

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The level of kynurenine hydroxylase was measured throughout the development of wild type and the eye color mutants v, cn, st, ltd, cd, kar, w, ca, bri and pP of Drosophila melanogaster. In all cases except cn a bimodal distribution of enzyme activity during development was observed. Activity is initially detectable in second instar. A maximum is reached in early third instar. Activity declines prior to puparium formation. Shortly after pupation, activity rises dramatically to reach a maximum about five times the peak larval level. Maximum activity persists for a short time, and then falls sharply prior to emergence. No activity is detectable in cn, cn3, or cn35K. In pupae which have zero, one, two or three doses of the cn  + allele, activity is proportional to the number of the + alleles. This provides further evidence that the cn locus contains the structural gene for kynurenine hydroxylase. Kynurenine hydroxylase is a useful gene product for studying the events of imaginal disc differentiation.