Ddc^de1, a Mutant Differentially Affecting Both Stage and Tissue Specific Expression of Dopa Decarboxylase in Drosophila

Clifton P. Bishop ¹ and Theodore R. F. Wright ¹

¹ Department of Biology, University of Virginia, Charlottesville, Virginia 22901

The isolation and characterization of a unique Dopa decarboxylase (Ddc) mutant in Drosophila melanogaster is reported. This mutant, Ddc^de1, exhibits stage- and tissue-specific altered Ddc expression. Homozygous Ddc^de1 embryos, central nervous systems (CNSs) at pupariation and newly eclosed adult epidermis all have approximately 5% as much specific dopa decarboxylase (DDC) activity as the pr control stock in which Ddc^de1 was induced. In contrast, the Ddc^de1 epidermis at pupariation has roughly 50% as much DDC activity as controls, a 10-fold increase over the relative activity detected in other tissues and stages. Although the adult cuticle lacks proper pigmentation as expected in flies with low DDC activity (5%), the bristles unexpectedly have wild-type black pigmentation. This implies that the bristle forming cells have more DDC activity than the rest of the adult epidermis. This variegated phenotype, black bristles and pale cuticle, plus the fact that Ddc^de1 was originally isolated in a reciprocal translocation between proximal X heterochromatin and the euchromatic left arm of the second chromosome, 42 bands from the Ddc locus, suggested that the mutant might be an example of position-effect variegation. All tests for position-effect variegation, including persistence of the mutant phenotype when Ddc^de1 was removed from the translocation, were negative. At pupariation DDC cross-reacting material (CRM) levels are similar in Ddc^de1 and wild-type controls, but in newly eclosed adults CRM levels are approximately 35% of wild-type controls. This suggests that DDC produced by Ddc^{de 1} adults has less activity per DDC molecule than the DDC produced at pupariation by Ddc^de1. If the DDC enzyme produced by Ddc^de1 at adult eclosion had full DDC activity (35% DDC CRM = 35% DDC activity) then no mutant phenotype would be exhibited by Ddc^de1 since flies with as little as 10% activity have a wild-type phenotype. DDC thermolability assays clearly demonstrate that DDC from Ddc^de1 is more thermolabile than control DDC at both pupariation and adult eclosion. Furthermore, DDC from adults in both Ddc^de1 and the pr control is more thermolabile than DDC from white prepupae. Mixing experiments indicate the difference in DDC thermolability between pr white prepupae and pr adults is not due to a difference in the white prepupal and adult supernatants. This suggests that in wild-type different isoforms of DDC are produced either by differences in post-translational modification or as a result of a different primary amino acid sequence. The lesion in Ddc^de1 may be in a translated sequence which is more important for proper Ddc expression in embryos, CNSs at pupariation and adult epidermis than it is in white prepupal epidermis and the bristle-forming cells.