Overlapping transcription units in the dopa decarboxylase region of Drosophila

The many examples of overlap in the genes of various viruses and bacteria illustrate that the parsimonious utilization of the coding capacity of DNA is relatively common amongst prokaryotes (for review, see ref. 1). The recent discovery of a pupal cuticle gene within an intron of the completely unrelated Gart locus in Drosophila2 shows that overlapping transcription units also exist in higher organisms. However, the prevalence of such phenomena is unknown. We report here a quite different situation of overlap between the 3′ termini of a pair of convergent transcription units in another region of the Drosophila genome. This 88-base-pair (bp) genomic region encodes the 3′ terminus of the messenger RNA for the enzyme dopa decarboxylase (Ddc) and, in opposite orientation, the 3′ terminus of the adjacent gene whose function is unknown. An analysis of the temporal and spatial distribution of the two transcripts within the organism shows that high levels of both transcripts are never concordant. However, within the testes, where the 3′ transcript is maximally expressed, low levels of Ddc transcript were detected. This result raises the possibility that a hybrid molecule involving the two transcripts forms in vivo or that transcription interference occurs, with concomitant regulatory implications.