Yeast RNA polymerase II (Pol II) general transcription factor TFIIE and the TFIIH subunit Ssl2 (yeast ortholog of mammalian XPB) function in the transition of the preinitiation complex (PIC) to the open complex. We show that the three TFIIE winged-helix (WH) domains form a heterodimer, with the Tfa1 (TFIIEα) WH binding the Pol II clamp and the Tfa2 (TFIIEβ) tandem WH domain encircling promoter DNA that becomes single-stranded in the open complex. Ssl2 lies adjacent to TFIIE, enclosing downstream promoter DNA. Unlike previous proposals, comparison of the PIC and open-complex models strongly suggests that Ssl2 promotes DNA opening by functioning as a double-stranded-DNA translocase, feeding 15 base pairs into the Pol II cleft. Right-handed threading of DNA through the Ssl2 binding groove, combined with the fixed position of upstream promoter DNA, leads to DNA unwinding and the open state.
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