TFIID complexes

The side of the p sheet that faces away from DNA is covered by two long a helices. One of these helices contains a number of basic residues from the middle segment of the polypeptide chain while the second helix is formed by the C-terminal residues. Residues from these two helices and from the short loop that joins the two motifs (red in Figure 9.4) are likely candidates for interactions with other subunits of the TFIID complex, and with specific transcription factors. 

Figure 37-9. The eukaryotic basal transcription complex. Formation of the basal transcription complex begins when TFIID binds to the TATA box. It directs the assembly of several other components by protein-DNA and protein-protein interactions. The entire complex spans DNA from position -30 to +30 relative to the initiation site (+1, marked by bent arrow). The atomic level, x-ray-derived structures of RNA polymerase II alone and ofTBP bound to TATA promoter DNA in the presence of either TFIIB or TFIIA have all been solved at 3 A resolution. The structure of TFIID complexes have been determined by electron microscopy at 30 A resolution. Thus, the molecular structures of the transcription machinery are beginning to be elucidated. Much of this structural information is consistent with the models presented here.

Fig. 1.34 Activators and coactivators of transcription intiation. The figure shows the function of three groups of proteins that function as coactivators. The general cofactors mediate the interactions between the specific transcription activators and the TFIID complex as well as with various forms of the RNA polymerase II holoenzyme. The TAFs are components of the TFIID complex and serve as contact points for specific transcription activators. The mediators are components of various forms of holoenzymes of RNA polymerase II. SRB proteins belong to the class of mediators, which, among other things, interacts with the CTD of RNA polymerase. The simphfied diagram does not show the interactions with chromatin.

In one model it is assumed that transcriptional activators and coactivators increase the efficiency of formation of the pre-initiation complex. This function includes a restructuring of chromatin at the transcription start site. In this context the formation of the TFIID complex at the promoter plays an important role. 

In the N-terminal region of p53, there is a transactivation domain which p53 uses to make contact with the transcription apparatus. Different protein binding sites have been identified in this region. These include binding sites for components of the TFIID complex and for coactivators such as the CBP/p300 coactivator (see 1.4.6). 

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. 

Interaction of the TATA-box-binding protein (TBP) with promoter DNA is rather inefficient and appears to be the rate-limiting step for the start of transcription. TBP must actually dissociate first from the TFIID complex before it can bind to the TATA-box DNA. Dissociation of TBP is facilitated by the dimeric structure of TFIID, when it is not bound to DNA, and by the interaction of TBP with TFIIA. 

Fortunately, the complicated process of the assembly of the many components of PIC at transcription initiation sites can be simplified by substituting the multicomponent TFIID complex by TBP. The TBP—TATA-box complex represents a reduced PIC which can actually carry out transcription in vitro. 

Finally, in Plate 21, the structure of the yeast TFIIA-TBP-DNA complex is shown. TFIIA interacts with TBP and the TFIID complex and stimulates transcription of the Pol II gene.29-32 TFIIA has two characteristic structural motife. One is a six-stranded (i-sand-wich, the other is a left-handed four-helix bundle. The P-sandwich domain of TFUA is alone responsible for all of the interactions with the DNA, whereas its helix-bundle domain projects away and is free to interact with signal-responsive transcription factors. These interactions are important for regulation of transcription. Little conformational change occurs in TBP when it binds to TFIIA. The main difference between the TFIIA-TPB complex and the TFIIB-TBP complex is that TFIIA binds upstream of the TATA box, away from the transcriptional start site, whereas TFIIB binds downstream of the TATA box. Moreover, TFIIB is positioned on the side opposite to TFIIA. 

Pugh, B. E, and Tjian, R. (1992). Diverse transcriptional functions of the multisubunit eukaryotic TFIID complex. J. Biol. Chem. 267, 679-682. 

The TAFs are components of TFIID (see Table 1.1) and are required for a regulated transcription (review Albright and Tijan, 2000). Thus, the stimulation of transcription by the transcriptional activators Spl and NTF-1 depends upon the presence of specific TAFs in the TFIID complex. The TAFs mediate interactions between the transcriptional activators and the TFIID complex in many cases direct protein-protein interactions could be demonstrated between the activators and TAFs. Some of the TAFs possess enzymatic activities which allow them to participate in the regulation of transcription. By this token, the histone acetylase and protein kinase activity of TAFn250 is ascribed a regulatory function in the remodeling of chromatin and in the control of the activity of the basal transcription factors. 

TFIIB interacts with the C-terminal stirrup of TBP and the DNA both upstream and downstream of the TATA box, whereas TBIIA interacts with the N-terminal stirrup and the DNA upstream of the TATA box on the opposite face of the double helix from TFIIB (Nikolov and Burley, 1997). Therefore, both TFIIA and TFIIB can bind to the TBP-DNA complex simultaneously and synergistically stabilize the complex. Furthermore, TFIIB binding also contributes to the directionality of transcription and forms a bridge between TBP and Pol II and specifies the transcription start site. It will be interesting to see how these interactions fit into the structure of the TFIID complex. 

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