Promoter activity profiling throughout the Drosophila life cycle reveals role of transposons in regulatory innovation

Since their discovery, transposable elements have been proposed to play a central role in the evolution of their host genomes through their ability to regulate gene expression, in particular by providing transcription start sites (TSSs) for host genes. To investigate their contribution to developmental gene expression, we developed RAMPAGE, a high-throughput 5'-complete cDNA sequencing approach to accurately discover TSSs, characterize their transcripts, and quantify their expression. This strategy, which directly delineates the expression profiles of individual promoters and was designed to offer optimal sample multiplexing capabilities, represents an advantageous alternative to standard RNA-Seq for a wide range of transcriptome profiling applications. We used RAMPAGE in a genome-wide study of promoter activity throughout 36 stages of the life cycle of Drosophila melanogaster, and describe here a comprehensive dataset that represents the first developmental timecourse of promoter usage. We found that over 40% of developmentally expressed genes have at least 2 promoters, and that alternative promoters generally implement distinct regulatory programs. Transposons harbor TSSs driving the expression of hundreds of annotated genes, and they often impart their own expression specificity upon the genes they regulate. Detailed analysis of particular transposons identified sequence elements encoding these regulatory properties. Our results show that transposable elements contribute significantly to the generation of standing variation and to the evolution of gene regulatory networks, by distributing stereotyped regulatory modules throughout the genome.