Abstract
Promoter-proximal pausing of RNA polymerase (Pol) II is a key regulatory step during transcription. Despite the central role of pausing in gene regulation, we do not understand the evolutionary processes that led to the emergence of Pol II pausing or its transition to a rate-limiting step actively controlled by transcription factors. Here, we analyzed transcription in species across the tree of life. Unicellular eukaryotes display an accumulation of Pol II near transcription start sites, which we propose transitioned to the longer-lived, focused pause observed in metazoans. This transition coincided with the evolution of new subunits in the negative elongation factor (NELF) and 7SK complexes. Depletion of NELF in mammals shifted the promoter-proximal buildup of Pol II from the pause site into the early gene body and compromised transcriptional activation for a set of heat-shock genes. Our work details the evolutionary history of Pol II pausing and sheds light on how new transcriptional regulatory mechanisms evolve.