A: Subunit Structure and Function

After formation of a TFIID-TFIIB-pol II/TFIIF-promoter complex, the next step in the sequential assembly pathway is the recruitment of TFIIE and TFIIH. TFIIE consists of 2 subunits, a and p, which form an a2p2 heterotetramer (Ohkuma et al, 1991; Sumimoto et al., 1991; Peterson et al, 1991). In humans, TFIIEa is the larger subunit consisting of 439 amino acids with a molecular weight of 56 kDa, while TFIIEp is 291 amino acids with an approximate molecular weight of 34 kDa. The N-terminal half of TFIIEa is necessary for interactions with TFIIEp and pol II via non-overlapping regions, for basal transcription, for stimulating TFIIH-mediated CTD phosphorylation, and for the transition from initiation to elongation, while the C-terminal region is involved in TFIIH interaction and appears nonessential in yeast (Ohkuma etal., 1995; Kuldell and Buratowski, 1997; Okuda et al., 2004). In analogy to TFIIF, the archaeal bacteria TFE protein, which shows sequence homology to the N-terminal half of TFIIEa but without the C-terminal domain, also has a winged HTH structure as revealed by x-ray crystallography (Meinhart et al., 2003). The NMR structure of the zinc-finger domain present in the N-terminal half of human TFIIEa different from the archaeal TFE region used for structural analysis, is comprised of one a-helix and five P-strands, which is distinct from conventional zinc finger structures (Okuda et al., 2004). In human TFIIEP, the C-terminal basic helix loop helix region possesses single-stranded DNA-binding activity (Okamoto et al., 1998). The central core domain (amino acids 66-146) of human TFIIEP, which contains amino acid residues homologous to the pol II-binding region of RAP30, also has a winged HTH structure (Okuda et al., 2000). Similar to the function of the TFIIEa N-terminal half, the TFIIEP C-terminus is also implicated in the transition from transcription initiation to elongation by pol II (Watanabe et al., 2003). It remains to be determined whether the TFIIEa N-terminal region and the TFIIEP C-terminal region function independently or cooperatively in aiding pol II transition from initiation to elongation.

B: TFIIE Function

As seen with other GTFs, TFIIE may be recruited to the promoter through direct interaction with gene-specific transcriptional activators (Sauer et al., 1995; Zhu and Kuziora, 1996). Once recruited, TFIIE interacts directly with both subunits of TFIIF, TFIIB, pol II, promoter DNA, and then recruits TFIIH (Flores et al., 1989; Maxon et al., 1994; Hampsey, 1998; Watanabe et al., 2003; Forget et al., 2004). TFIIE binds to pol II near its active center and to the promoter DNA approximately 10 bp upstream from the transcription initiation site, where promoter melting begins (Douziech et al, 2000; Forget et al., 2004). TFIIE can stimulate the ATPase, CTD kinase, and DNA helicase activities of TFIIH and thus facilitates the formation of an initiation-competent pol II complex (Ohkuma, and Roeder, 1994; Serizawa et al., 1994; Ohkuma et al., 1995; Lee and Young, 2000). Essential for the transition from PIC assembly to transcription initiation are the participation of TFIIE and TFIIH in promoter melting (Holstege et al., 1996). Consistent with TFIIE's role in promoter melting is its ability to bind single-stranded

DNA (Kuldell and Buratowski, 1997). Interestingly, requirement of TFIIE and TFIIH also depends on DNA topology and the promoter sequence (Parvin and Sharp, 1993; Goodrich and Tjian, 1994; Wu et al., 1998), consistent with the observation that TFIIE and TFIIH are not necessary for transcription from premelted promoter templates (Pan and Greenblatt, 1994; Holstege etal., 1996).

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