Widespread backtracking by RNA pol II is a major effector of gene activation, 5’ pause release, termination and transcription elongation rate

In addition to phosphodiester bond formation, RNA polymerase II has an RNA endonuclease activity, stimulated by TFIIS, which rescues complexes that have arrested and backtracked. We identified backtracking sites using mNET-seq in human cells expressing dominant-negative TFIIS (TFIISDN) that inhibits RNA cleavage and stabilizes backtracked complexes. Backtracking is most frequent within 2 kb of start sites, consistent with slow elongation early in transcription, and in 3’ flanking regions where termination is enhanced by TFIISDN. Inhibition of the rescue from backtracked pol II by TFIISDN impaired the escape from 5’ pause sites, the completion of long transcripts, and activation of stress-inducible genes. TFIISDN slowed elongation rates genome-wide by half as measured by sequencing nascent transcripts pulse labeled with Bromouridine (Bru-seq) and by anti-pol II ChIP-seq at time points after release of a DRB block. These results suggest that rescue of backtracked pol II by TFIIS is a major stimulus of elongation. 66 samples