An analysis of dopa decarboxylase expression during embryogenesis in Drosophila melanogaster

Dopa decarboxylase (DDC) activity appears near the end of embryogenesis in Drosophila. High titers of 20-OH-ecdysone on the other hand are found at midembryogenesis. Several explanations for this lag were investigated, since this hormone has been shown to induce a rapid increase in DDC activity at pupariation (G. P. Kraminsky, W. C. Clark, M. A. Estelle, R. D. Gietz, B. A. Sage, J. D. O'Connor, and R. B. Hodgetts, 1980, Proc. Natl. Acad. Sci. USA 77, 4175-4179). Using immunological and genetical criteria, it was shown that the same structural gene encodes DDC in embryos, mature larvae, and young adults. This rules out the existence of a distinct embryonic DDC gene unresponsive to 20-OH-ecdysone. Second, no evidence was found to support the hypothesis that a delay in the translation of DDC transcripts, produced in response to the elevated titer of 20-OH-ecdysone at midembryogenesis, caused the lag. Northern analysis of the RNA molecules homologous to cloned genomic sequences revealed that DDC transcripts were present at two different times during embryogenesis. A transcript was found in both ovaries and 0- to 2-hr embryos. However, this species disappeared by 4 hr and DDC transcript levels remained low until late in embryogenesis, when a significant increase occurred. This increase was presumably responsible for the appearance of the enzyme at this time. The northern blotting revealed nine DDC transcript species were present during embryogenesis and hybridization to intron-specific probes indicated that five of these contained at least part of one (or both) of the two introns. Three putative mature mRNA species were identified by their small size, relative abundance, apparent lack of intron sequence, and their presence on polysomes. The two mature species found during the late stages were postulated to differ in the length of their poly(A)+ tails. The third mature species was found only in ovaries and very young embryos and may well be of maternal origin. Data are examined in light of the possibility that this species is derived from a precursor initiated at a novel promotor.