TFIIF formation

TBP bound to the TATA box is the heart of the initiation complex (see Figure 28.19). The surface of the TBP saddle provides docking sites for the binding of other components (Figure 28.21). Additional transcription factors assemble on this nucleus in a defined sequence. TFIIA is recruited, followed by TFIIB and then TFIIF—an ATP-dependent helicase that initially separates the DNA duplex for the polymerase. Finally, RNA polymerase II and then TFIIE join the other factors to form a complex called the basal transcription apparatus. Sometime in the formation of this complex, the carboxyl-terminal domain of the polymerase is phosphorylated on the serine and threonine residues, a process required for successful initiation. The importance of the carboxyl-terminal domain is highlighted by the finding that yeast containing mutant polymerase II with fewer than 10 repeats is not viable. Most of the factors are released before the polymerase leaves the promoter and can then participate in another round of initiation. 

Transcription initiation begins by the formation of a preinitiation complex, and most of the control of transcription occurs at this step. This complex normally contains RNA polymerase II and six general transcription factors (GTFs)—TTHA, TFIIB, TTIID, TFIIE, TFIIF, and TFIIH. 

Pol II does not elongate efficiently when alone in vitro. Under those circumstances, it can synthesize only 100-300 nucleotides per minute, whereas the in vivo rates are between 1500 and 2000 nucleotides per minute. The difference is due to elongation factors. One is TFIIF, which, in addition to its role in the formation of the preinitiation complex, also has a separate stimulatory effect on elongation. A second elongation factor, which was named TFIIS, was more recently discovered. 

Kimura, M., Suzuki, H., and Ishihama, A. (2002). Formation of a carboxy-terminal domain phosphatase (Fcpl)/TFIIF/RNA polymerase II (pol II) complex in Schi-zosaccharomyces pombe involves direct interaction between Fepl and the Rpb4 subunit of pol II. Mol. Cell. Biol. 22(5), 1577-1588. 

---