SummaryThe generation of distinct messenger RNA isoforms through alternative RNA processing modulates the expression and function of genes, often in a cell-type-specific manner. Here, we assess the regulatory relationships between transcription initiation, alternative splicing, and 3′ end site selection. Applying long-read sequencing to accurately represent even the longest transcripts from end to end, we quantify mRNA isoforms inDrosophilatissues, including the transcriptionally complex nervous system. We find that inDrosophilaheads, as well as in human cerebral organoids, 3′ end site choice is globally influenced by the site of transcription initiation (TSS). “Dominant promoters,” characterized by specific epigenetic signatures including p300/CBP binding, impose a transcriptional constraint to define splice and polyadenylation variants.In vivodeletion or overexpression of dominant promoters as well as p300/CBP loss disrupted the 3′ end expression landscape. Our study demonstrates the crucial impact of TSS choice on the regulation of transcript diversity and tissue identity.