Human RNA Polymerase II Elongation in Slow Motion: Role of the TFIIF RAP74 α1 Helix in Nucleoside Triphosphate-Driven Translocation

The role of the RAP74 α1 helix of transcription factor IIF (TFIIF) in stimulating elongation by human RNA polymerase II (RNAP II) was examined using millisecond-phase transient-state kinetics. RAP74 deletion mutants RAP74(1-227), which includes an intact α1 helix, and RAP74(1-158), in which the α1 helix is deleted, were compared. Analysis of TFIIF RAP74-RAP30 complexes carrying the RAP74(1-158) deletion reveals the role of the α1 helix because this mutant has indistinguishable activity compared to TFIIF 74(W164A), which carries a critical point mutation in α1. We report adequate two-bond kinetic simulations for the reaction in the presence of TFIIF 74(1-227) + TFIIS and TFIIF 74(1-158) + TFIIS. TFIIF 74(1-158) is defective because it fails to promote forward translocation. Deletion of the RAP74 α1 helix results in increased occupancy of the backtracking, cleavage, and restart pathways at a stall position, indicating reverse translocation of the elongation complex. During elongation, TFIIF 74(1-158) fails to support detectable nucleoside triphosphate (NTP)-driven translocation from a stall position and is notably defective in supporting bond completion (NTP-driven translocation coupled to pyrophosphate release) during the processive transition between bonds.