Abstract
The enzyme alcohol dehydrogenase of Drosophila melanogaster is encoded by a single structural gene (Adh) with two promoters, distal and proximal (PD and PP). During development these two promoters are used differently: the major Adh transcript of larvae is from PP, the major transcript of adult flies is from PD. At a few discrete times in development transcription occurs simultaneously from both promoters. In situ hybridization has been used to investigate the spatial and temporal aspects of promoter activity at these stages of development. Maternally inherited Adh transcripts are not localized in the embryo; they decay very rapidly after fertilization. Zygotic expression of Adh RNA begins after germ-band retraction, 10.5 hr after fertilization. Expression is confined to the fat body, but occurs from both distal and proximal promoters. By 15 hr expression is first seen in the gut, from PP. By the same time fat body expression from PD has ceased, and transcription in this tissue is exclusively from PP for the next 4 days. The steady-state level of Adh transcript begins to decline at the end of larval development. There is then the transient accumulation of transcripts from PD, but predominantly in the larval fat body, rather than in the gut. These data illustrate a surprising complexity in the tissue and temporal regulation of Adh expression in D. melanogaster. Moreover, they show that transcripts from two different promoters of the same gene can, at certain well-defined stages of development, accumulate in the same cells.
