Genome binding/occupancy profiling by high throughput sequencing
Paused RNA polymerase II that piles up near most human promoters is the target of mechanisms that control entry into productive elongation. Whether paused pol II is a stable or a dynamic target remains unresolved. We report that most 5’-paused pol II throughout the genome is turned over within 2 minutes. This process is revealed under hypertonic conditions that prevent pol II recruitment to promoters. This turnover requires cell viability but is not prevented by inhibiting transcription elongation suggesting that it is mediated at the level of termination. When initiation was prevented by triptolide during recovery from high salt, a novel pre-initiated state of pol II lacking the pausing factor Spt5 accumulated at transcription start sites. We propose that pol II occupancy near 5’ ends is governed by a cycle of ongoing assembly of pre-initiated complexes that transition to pause sites followed by eviction from the DNA template. This model suggests that mechanisms regulating the transition to productive elongation at pause sites operate on a dynamic population of pol II that is turning over at rates far higher than previously suspected. We suggest that a plausible alternative to elongation control via escape from a stable pause, is by escape from premature termination.
Initiation of transcription was inhibited by high salt treatment or treatment with triptolide and pol II occupancy was mapped around start sites and within genes by anti-pol II ChIP-seq in HCT116 cells at time points between 2 and 10 minutes afterwards.