TBP/TAF complexes

Goodrich, J.A., Tjian, R. TBP TAF complexes selectivity factors for eucaryotic transcription. Curr. Opin. Cell Biol. 6 403-409, 1994. 

Furthermore, a multisubunit TAF complex was purified from yeast extracts using GST-TBP as a ligand for affinity chromatography (Reese et al., 1994). The use of epitope tag and immunoprecipitation also led to the isolation of TBP-TAFs complex from yeast extract (Poon et al.,... 

The general transcription factor TFllD is believed to be the key link between specific transcription factors and the general preinitiation complex. However, the purification and molecular characterization of TFllD from higher eucaryotes have been hampered by its instability and heterogeneity. All preparations of TFllD contain the TATA box-binding protein in combination with a variety of different proteins called TBP-associated factors, TAFs. When the preinitiation complex has been assembled, strand separation of the DNA duplex occurs at the transcription start site, and RNA polymerase II is released from the promoter to initiate transcription. However, TFIID can remain bound to the core promoter and support rapid reinitiation of transcription by recruiting another molecule of RNA polymerase. 

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.

This consists of DNA-dependent RNA polymerase II and basal transcription factors (TFIIX, X = A - H). First, the basal factor TFIID binds to the promoter. TFIID, a large complex of numerous proteins, contains TATA boxbinding protein (TBP) and so-called TAFs (TBP-associated factors). The polymerase is attached to this core with the help of TFIIB. Before transcription starts, additional TFs have to bind, including TFIIH, which has heli-case activity and separates the two strands of DNA during elongation. In all, some 35 differ-... 

Direct inhibition of the formation of a pre-initiation complex complexation of basal transcription factors, such as TFIID or TFIIB, or competition with TFIIB for binding to the promoter. An example for this type of repression is the negative cofactor NC2 (see 1.4.3.2). Transcription repression can also result from phosphorylation of the basal transcription factors. By this token, the repression of transcription observed during mitosis is attributed to the hyperphosphorylation of TBP and TAFs. 

The best-characterized coactivator is the transcription factor TFIID (Fig. 28-27). In eukaryotes, TFIID is a large complex that includes TBP and ten or more TBP-associated factors (TAFs). Some TAFs resemble histones and may play a role in displacing nucleosomes during the activation of transcription. Many DNA-binding transactivators aid in transcription initiation by interacting with one or more TAFs. The requirement for TAFs to initiate transcription can vary greatly from one gene to another. Some promoters require TFIID, some do not, and some require only subsets of the TFIID TAF subunits. 

TATA box-binding protein-associated factor 10, a component of the general transcription factor complex TFllD and the TATA box-binding protein (TBP)-free TAF-containing complex... 

Fig. 1.21 I nitial protein complexes formed at the TATA box and the initiation region Selection of the transcription start site is mediated either by the TATA box or the initiation region INR. The TATA box is bound by the TATA box binding protein TBP in cooperation with the general transcription factors IIA and MB. In the absence of a TATA box, transcription initiation requires binding of the TAF 250 and TAFM150 to the INR.

Initiation complex - All of the protein-coding genes in eukaryotes are transcribed by RNA polymerase II (pol II) This enzyme also transcribes some of the small nuclear RNAs involved in splicing (see here). Like other RNA polymerases, pol II is a complex, multisubunit enzyme, but not even its numerous subunits are sufficient to allow pol II to initiate transcription on a eukaryotic promoter. To form a minimal complex capable of initiation, at least five additional protein factors are needed Figure 28.24 and listed in Table 28.4. The minimal unit involves the TATA binding protein, (TBP), but in vivo formation of the complex probably always uses TFllD, a multi-subunit structure incorporating both TBP and TATA binding associated factors (TAFs). 

A number of the core transcription factors are released and RNA polymerase II, together with TFIIF, moves along the DNA. A residual complex, containing the TATA binding protein (TBP), TFIIA, TATA binding associated factors (TAFs) and probably activator proteins, remains at the start site, ready to initiate another round of transcription. 

TFIID is one of the general transcription factors for RNA polymerase II. Part of it is a protein that binds to the TATA box in eukaryotic promoters. Associated in complex with the TATA box and the TBP are many proteins called TAFs, for TBP associated factors. 

---