TFIIE

Figure 37-10. Two models for assembly of the active transcription complex and for how activators and coactivators might enhance transcription. Shown here as a small oval is TBP, which contains TFIID, a large oval that contains all the components of the basal transcription complex illustrated in Figure 37-9 (ie, RNAPII andTFIIA,TFIIB, TFIIE,TFIIF, and TFIIFI). Panel A The basal transcription complex is assembled on the promoter after the TBP subunit of TFIID is bound to the TATA box. Several TAFs (coactivators) are associated with TBP. In this example, a transcription activator, CTF, is shown bound to the CAAT box, forming a loop complex by interacting with a TAF bound to TBP. Panel B The recruitment model. The transcription activator CTF binds to the CAAT box and interacts with a coactivator (TAF in this case). This allows for an interaction with the preformed TBP-basal transcription complex. TBP can now bind to the TATA box, and the assembled complex is fully active.

TFIIE binds TFIIH to assist the latter with the melting of the promoter. 

General transcription initiation factors TFIIB, TFIIE, TFIIF and TFIIH have been identified as components of the RNA polymerase 11 holoenzyme of yeast. Various forms of the yeast holoenzyme contain further proteins, known as mediators or SRB proteins (SRB, suppressor of RNA polymerase B). The mediators fimction as coactivators (see 1.4.3.2). The holoenzyme is difficult to define structurally because the proteins accessory to the core enzyme (see table 1) may not be permanently associated with RNA polymerase II. 

TFIIE 2 34,000, 57,000 Recruits TFIIH hasATPase and helicase activities... 

During synthesis of the initial 60 to 70 nucleotides of RNA, first TFIIE and then TFIIH is released, and Pol II enters the elongation phase of transcription. 

TATA-Binding Protein The first component to bind in the assembly of a preinitiation complex at the TATA box of a typical Pol II promoter is the TATA-binding protein (TBP). The complete complex includes the basal (or general) transcription factors TFIIB, TFIIE, TFIIF, TFIIH Pol II and perhaps TFIIA (not all of the factors are shown in Fig. 28-27). This minimal preinitiation complex, however, is often insufficient for the initiation of transcription and generally does not form at all if the promoter is obscured within chromatin. Positive regulation leading to transcription is imposed by the transactivators and coactivators. 

Phil Sharp and Leonard Guarente showed that at least four transcription factors are required in addition to polymerase II for initiation from the major late promoter of adenovirus. In vitro studies indicate that these factors assemble in an orderly fashion (see fig. 28.12b). First the TFIID complex binds to the TATA box. Sequential binding of TFIIA, TFIIB, RNA polymerase II, and TFIIE follow. It is believed that this multifactor complex functions for a large number of eukaryotic promoters that contain TATA boxes. 

Fig. 3. Initiation of transcription by RNA polymerase II. TFIID binds to the TATA box followed in order by the binding of TFIIA, TFIIB and a pre-formed complex of TFIIF. RNA polymerase II. Subsequently TFIIE, TFIIH and TFIIJ bind in order and transcription then starts about 25 bp downstream from the TATA box. Note that the placement of the various factors in this diagram is arbitrary their exact positions in the complex are not yet known.

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. 

Several families of transcription factors exist. These include basal (or general) transcription factors, activators, and repressors. The basal transcription factors include transcription factor IIA (TFUA), TFIIB, TRID, TFIIE, TRIP, and TFllH. Most of these transcription factors exist as multiprotein complexes. These transcription factors must be assembled just upstream of the transcribed part of the gene before RNA polymerase begins its catalytic activity. When assembled, the entire "mega-complex" consists of about 50 proteins (Jacobson and Tjian, 1996). 

TFIIE 2 34, 57 binding of TFIIH, modulation of activities of TFIIH... 

TFIIF is found in a pre-formed complex with RNA polymerase II and suppresses the nonspecific binding of RNA polymerase to DNA. TFIIF supports the association of RNA polymerase with the promotor-bound complex of TFIIA, TFIIB and TFIID. TFIIE binds TFIIH to assist the latter with the melting of the promotor. 

Holoenzyme forms of RNA polymerase II have general transcription factors associated. TFIIB, TFIIE, TFIIF and TFIIH have been identified as components of the RNA polymerase II holoenzyme of yeast. 

Before RNA polymerase II can begin transcribing a gene, transcription factors must assemble into a complex. The process begins when TFIID binds to a TATA sequence. TFIID, which consists of TBP (a TATA binding protein) and several associated proteins, binds to and unwinds the DNA duplex in the TATA sequence. Then TFHB binds and, later, TFIIE, TFIIH, and TFIU. TFIIF binds directly to RNA polymerase n. Because of phosphorylation reactions catalyzed by TFIIH, the RNA polymerase II becomes active and begins transcription. 

Park, C.H., Mu, D., Reardon, J.T., and Sancar, A. (1995) The general transcription-repair factor TFIIH is recruited to the excision repair complex by the XPA protein independent of the TFIIE transcription factor. /. Biol. Chem., 270, 4896-4902. 

Transcription of protein-coding genes by Pol II can be initiated in vitro by sequential binding of the following in the indicated order TBP, which binds to TATA-box DNA TFIIB a complex of Pol II and TFIIF TFIIE and finally TFIIH (see Figure 11-27). 

TFIIE, TFIIF, TFIIH, TFIIJ, and TFIIA. Phosphorylation of the carboxy terminal repeat domain (CTD) of the RNA pol II large subunit is associated with initiation of RNA synthesis. 

Catalytic functions in RNA synthesis recruitment of TFIIE TFIIH recruitment imxliilation of TFIIH helicase, ATPitse, and kinase activities ... 

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