The role of specific enhancer-promoter interactions in the Drosophila Adh promoter switch

The Drosophila melanogaster alcohol dehydrogenase (Adh) gene is transcribed from two promoters active at different developmental stages. In this paper we show that the promoters are differentially stimulated by two enhancers, the Adh larval enhancer and the Adh adult enhancer. In early larval stages, the larval enhancer stimulates transcription from the proximal promoter; in late larval stages, the two enhancers act synergistically to stimulate transcription from the distal promoter; and in adults, the adult enhancer stimulates transcription from the distal promoter. To determine the basis for these enhancer-promoter interactions, we examined the effect of each enhancer on three different promoters. We found that the adult enhancer is stage specific and stimulates transcription from all three promoters. In contrast, the larval enhancer is potentially active in all stages and stimulates transcription from only two of the three promoters. These observations suggest that normal temporal expression of Adh depends on the stage-specific activity of the adult enhancer and the differential response of the proximal and distal promoters to the larval enhancer.