Role of transcriptional interference in the Drosophila melanogaster Adh promoter switch

The Drosophilia melanogaster alcohol dehydrogenase (Adh) gene is transcribed from two closely linked promoters, which are regulated by two upstream enhancers. The proximal promoter is active primarily in first to early third-instar larvae, whereas the distal promoter is active in late third-instar larvae and adults. The Adh larval enhancer and the proximal promoter are separated by the Adh adult enhancer and the distal promoter. Because the proximal promoter is turned off just as the distal promoter is turned on, we considered the possibility that the distal promoter or adult enhancer has a role in the downregulation of the proximal promoter. We report here that transcription from the distal promoter is required to shut off the proximal promoter. In the absence of the distal promoter, the proximal promoter is active throughout larval development and in adults. The proximal promoter is also aberrantly active in adults when placed upstream of the distal promoter. These results suggest that the developmental switch from proximal to distal promoter is regulated by the stage-specific activation of the distal promoter, and the subsequent repression of the proximal promoter by transcriptional interference.