Initiation factor Basal

The TATA box and/or an initiation sequence are structural elements which define a minimal promoter from which in vitro transcription can be initiated. A classical TATA box is often, though not always, ca. 30bp from the transcription start site. The initiation sequence includes sequences in the immediate vicinity of the transcription start site. The TATA box and initiation sequence are sufficient for the formation of a basal transcription apparatus composed of general initiation factors for transcription and RNA polymerase II (see Fig. 1.31). 

Promoter selection, binding of basal initiation factors... 

Formation of a Basal Transcription Apparatus from General Initiation Factors and RNA Polymerase... 

In contrast to the procaryotes, where the o -holoenzyme of the RNA polymerase can initiate transcription without the aid of accessory factors, the eucaryotic RNA polymerase requires the help of numerous proteins to begin transcription. These proteins are termed basal or general initiation factors of transcription. Together with RNA polymerase II, they participate in the basal transcription apparatus. The various components must associate in a defined order for the formation of a transcription-competent complex, from which a low level of transcription is possible. An increase in the basal transcriptional level requires the effect of specific transcriptional activators, which bind cognate DNA sequences at a variable distance from the promoter. The transcriptional activators themselves require the aid of further protein factors, known as coactivators (see 1.4.3.2), in order to attain full stimulatory activity. 

The purification and structural and functional characterization of the general initiation factors has proven extremely difficult. The specific function of the various factors, as well as their structural role in the entire complex, remains poorly resolved. According to the current model, the general transcription initiation factors, with which an exact start of transcription is possible in vitro, are required for the formation of a basal... 

The formation of an active, regulation-competent initiation complex for transcription in eucaryotes demands the concerted action of a large number of proteins. It is estimated that more than 50 different proteins participate in the initiation of transcription in eucaryotes. The basal transcription complex, consisting of the general initiation factors, as well as RNA polymerase 11, allows only for a slow transcription rate. For a regulated acceleration of this low transcription rate it is necessary to have - apart from the regulatory DNA-binding proteins - mediation by further co-activator proteins. 

The TATA binding protein and general transcription initiation factors. A slow basal level of transcription can be observed when all but a small part of the control region at the 5 end of a gene is deleted.325 This minimum promoter, which includes the TATA sequence, is the binding site of both the RNA... 

The entire top side of TBP bound to the TATA-box DNA is out of the way and is fi e to interact with other factors. This leaves a generous surface on TBP available for interactions with the multitude of factors which are parts of the transcription initiation complex. The same is true for the TFIIA/TFIIB-TBP-complexes. The extensive surfaces displayed by the TFIIA/TFIIB-TBP-DNA complexes represent potential sites for binding basal initiation factors, signal-responsive transcriptional activators, co-activators and mediators, and, most importantly, leave room for Pol II. 

Promotor selection, binding of basal initiation factors Binding of the RNA polymerase II Formation of a basal transcription apparatus... 

In contrast to the procaryotes, where the er70-holoenzyme of the RNA polymerase can initiate transcription without the aid of accessory factors, the eucaryotic RNA polymerase requires the help of numerous proteins to begin transcription. These proteins are termed basal or general initiation factors oftranscription. Together with RNA polymerase... 

Abbreviations APC anaphase-promoting complex ARC AAA ATPase-forming ring-shaped complexes eIF3 eukaryotic initiation factor 3 PAN proteasome-activating nucleotidase TFIIH basal transcription factor IIH. 

Sequences farther upstream from the start site determine how frequently the transcription event occurs. Mutations in these regions reduce the frequency of transcriptional starts tenfold to twentyfold. Typical of these DNA elements are the GC and CAAT boxes, so named because of the DNA sequences involved. As illustrated in Figure 37—7, each of these boxes binds a protein, Spl in the case of the GC box and CTF (or C/EPB,NF1,NFY) by the CAAT box both bind through their distinct DNA binding domains (DBDs). The frequency of transcription initiation is a consequence of these protein-DNA interactions and complex interactions between particular domains of the transcription factors (distinct from the DBD domains—so-called activation domains ADs) of these proteins and the rest of the transcription machinery (RNA polymerase II and the basal factors TFIIA, B, D, E, F). (See... 

In eukaryotes, general transcription factors must bind to the promoter to allow RNA polymerase II to bind and form the initiation complex at the start site for transcription. General manscription factors are common to most genes. The general transcription factor TFIID (the TATA fector) must bind to the TATA box before RNA polymerase II can bind. Other examples delude SP-1 and NF-.l that modulate basal transcription of many genes. 

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