High levels of intron-containing RNAs are associated with expression of the Drosophila DOPA decarboxylase gene.

We have examined the structure and expression during embryonic development of the Drosophila DOPA decarboxylase gene, Ddc. The Ddc gene is transcribed to make at least five different size classes of RNA. These RNA species first appear late in embryogenesis, coincident with induction of Ddc enzyme activity. The most abundant and smallest RNA appears to be Ddc mRNA. The sequences encoding this RNA are split by two intervening sequences. Each of the larger RNA species contains some or all of the intervening sequences. We have noted two unusual features of Ddc expression during embryogenesis. First, the intervening-sequence-containing RNAs are present as 20% or more of the polyadenylated Ddc RNA molecules, an exceptionally high proportion. Second, these RNAs do not disappear as rapidly as Ddc mRNA after Ddc enzyme activity reaches fully induced levels. These observations indicate slow rates of RNA processing relative to mRNA half-life and suggest that post-transcriptional steps participate in regulating Ddc expression. Although four of the five RNA species were detected at multiple developmental stages during which Ddc is expressed, one was found uniquely during embryogenesis. This RNA differs from Ddc mRNA in length and in time of expression during embryogenesis but is transcribed in the same orientation and from the same genomic sequences as the Ddc primary transcript.