Abstract
Eukaryotic RNA polymerase II (Pol II) has been found at both promoters and distal enhancers, suggesting additional functions beyond mRNA production. To understand this role, we sequenced nascent RNAs at single-molecule resolution to unravel the interplay between Pol II initiation, capping and pausing genome-wide. Our analyses identify two pause classes that are associated with different RNA capping profiles. More proximal pausing is associated with less complete capping, less elongation and a more enhancer-like complement of transcription factors than later pausing. Unexpectedly, transcription start sites (TSSs) are predominantly found in constellations composed of multiple divergent pairs. TSS clusters are intimately associated with precise arrays of nucleosomes and correspond with boundaries of transcription factor binding and chromatin modification at promoters and enhancers. TSS architecture is largely unchanged during the dramatic transcriptional changes induced by heat shock. Together, our results suggest that promoter- and enhancer-associated Pol II is a regulatory nexus for integrating information across TSS ensembles.