Sequencing short capped RNAs captures acute transcription initiation and identifies promoter and distal regulatory elements across eukaryotes from total RNA

The spatial and temporal regulation of transcription initiation is pivotal for controlling gene expression. Here, we introduce capped-small RNA-seq (csRNA-seq), which uses total RNA as starting material to detect transcription start sites (TSS) of both stable and unstable RNAs at single-nucleotide resolution. csRNA-seq is highly sensitive to acute changes in transcription and identifies an order of magnitude more regulated transcripts than RNA-seq. Interrogating tissues from species across the eukaryotic kingdoms identified unstable transcripts resembling enhancer RNAs, pre-miRNAs, antisense transcripts and promoter upstream transcripts in multicellular animals, plants and fungi spanning 1.6 million years of evolution. Integration of epigenomic data from these organisms revealed that histone H3 trimethylation (H3K4me3) was largely confined to TSS of stable transcripts, while H3K27ac marked nucleosomes downstream of all active TSS, suggesting an ancient role for post-translational histone modifications in transcription. Our findings demonstrate that total RNA is sufficient to identify transcribed regulatory elements and capture the dynamics of initiated stable and unstable transcripts at single nucleotide resolution in eukaryotes.