The open region (“transcription bubble”) expands concomitant with the site of RNA-extension. In this case this region spans positions -9 to +3.
Goodrich, JA, Kugel, JF
Fiedler, U, Timmers, HT, Holstege, FC
DNA-directed RNA polymerase activity of RNA Polymerase II (unphosphorylated):TFIIF complex [nucleoplasm]
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