Structural Basis of Transcription: Backtracked RNA Polymerase II at 3.4 Angstrom Resolution

Abstract

Stepping Back to Go Forward Insight into the mechanism of transcription has come from crystal structures of actively transcribing RNA polymerase II complexes in the pre- and posttranslocation states. RNA polymerase also backtracks on the DNA template. Backtracking by only a few residues is reversible, but longer backtracking leads to arrest that is relieved by cleavage of the transcript by the transcription elongation factor SII (TFIIS). Now Wang et al. (p. 1203 ) report x-ray structures of backtracked ternary complexes and of a backtracked complex bound to a noncleaving mutant of TFIIS. The structures show a defined one-residue, backtracked state supporting the idea that RNA polymerase oscillates between backward and forward motion during active transcription. Mismatched residues disfavor forward translocation, increasing the lifetime of the backtracked state and facilitating cleavage by TFIIS. Thus, TFIIS-induced cleavage is likely to provide an important proofreading function during transcription.