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. How TFIIS affects transcription under normal conditions is poorly understood. We identified backtracking sites in human cells using a dominant-negative TFIIS (TFIIS) 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 TFIIS， suggesting that?backtracked pol II is a favorable substrate for?termination. Rescue from backtracking by RNA cleavage also promotes escape from 5' pause sites， prevents premature termination of long transcripts， and enhances activation of stress-inducible genes. TFIIS slowed elongation rates genome-wide by half， suggesting that rescue of backtracked pol II by TFIIS is a major stimulus of elongation under normal conditions.