Topoisomerase 1 activity during mitotic transcription favors the transition from mitosis to G1 [ChIP-seq]

By following chromatin occupancy of RNAPII and TOP1 using ChIP-seq throughout mitosis, we found that TOP1 is required for RNAPII translocation along genes. The stimulation of TOP1 activity through its interaction with RNAPII during elongation, allowed RNAPII clearance from genes in prometaphase and enabled proper chromosomal segregation. Interference with the TOP1-RNAPII interaction or acute depletion of TOP1 at the onset of mitosis impaired RNAPII spiking at promoters, which is necessary for the first pioneering round of transcription during mitotic exit, and triggered defects in the transcriptional program of the post-mitotic cells. ChIP-seq of RNAPII, Top1 and H3K4me3 throughout mitosis at three different timepoints: early, mid and late (PM, 0 and 40/60). To synchronize in mitosis cells were blocked in S phase by addition of 2.5mM thymidine (Sigma, T9250) for 19-24h and, after washing with fresh medium, transferred in fresh medium containing 400ng/ml nocodazole (Sigma, M1404) and allowed to progress to mitosis. Mitotic cells were collected and released to different mitotic stages before harvest and fixation for ChIP. All experiments were performed in HCT116 and derived cell lines.