In bacteria, a σ-factor–polymerase holoenzyme complex, Eσ, selectively and directly binds to promoter DNA to form the PIC. The situation is much more complex in the case of eukaryotic gene transcription. mRNA-encoding genes, which are transcribed by pol II, are described as an example. In the case of pol II–transcribed genes, the function of σ-factors is assumed by a number of proteins. Thus, accurate, specific PIC formation requires both RNA Pol II and the six GTFs (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH). These GTFs serve to promote RNA polymerase II transcription on essentially all genes. As noted earlier, a number of the GTFs are composed of multiple subunits. The TFIID complex (TFIID consists of 15 subunits, the TATA-box binding protein-TBP, and 14 TBP associated factors; TAFs) binds to the TATA box promoter element through its TBP and TAF subunits. TFIID is the only GTF that is independently capable of specific, high affinity binding to promoter DNA.

TBP binds to the TATA box in the minor groove of DNA (most transcription factors bind in the major groove) and causes an approximately 100-degree bend of the DNA helix. This bending is thought to facilitate the interaction of TFIID TAF subunits with other components of the transcription initiation complex, the multicomponent eukaryotic promoter, and possibly with factors bound to upstream elements. Although initially defined as a component solely required for transcription of pol II gene promoters, TBP, by virtue of its association with dis tinct, polymerase-specific sets of TAFs, is also an important com ponent of pol I and pol III transcription initiation complexes even if they do not contain TATA boxes.

The binding of TFIID marks a specific promoter for transcription. Of several subsequent in vitro steps, the first is the binding of TFIIA, then TFIIB to the TFIID-promoter com plex. Binding of TFIID, TFIIA, and TFIIB results in a stable multiprotein-DNA complex, one that is more precisely located and more tightly bound at the transcription initiation site. This complex attracts and tethers the pol II and TFIIF complex to the promoter. Addition of TFIIE and TFIIH are the final steps in the assembly of the PIC. Each of these binding events extends the size of the complex so that finally about 60 bp (from −30 to +30 relative to the +1 TSS) are covered (see Figure 1). The PIC is now complete and capable of basal transcription initiated from the correct +1 template-directed nucleotide. In genes that lack a TATA box, the same factors are required. In such cases, the Inr and/or DPE serve to position the complex for accurate initiation of transcription.

Fig1. The eukaryotic basal transcription complex. Formation of the basal transcription complex begins when TFIID binds, via its TATA binding protein (TBP) subunit and several of its 14 TBP-associated factor (TAF) subunits, to the TATA box. TFIID then directs the assembly of several other components by protein-DNA and protein–protein interactions: TFIIA, B, E, F, H, and polymerase II (pol II). The entire complex spans DNA from about positions −30 to +30 relative to the TSS at +1 (marked by bent arrow). The atomic level, X-ray–derived structures of RNA polymerase II alone and of the TBP subunit of TFIID bound to TATA promoter DNA in the presence of either TFIIB or TFIIA have all been solved at 3-Å resolution. The structures of mammalian and yeast basal transcription complexes (aka preinitiation complexes, or PICs) have also recently been determined at 10-Å resolution by electron microscopy. Thus, the molecular structures of the transcription machinery in action are beginning to be elucidated. Much of this structural information is consistent with the models presented here.

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قسم الشؤون الفكرية ينظم برنامجًا دينيًّا لكشافة الكفيل في النجف الأشرف

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دراسة: كوب ميلك شيك واحد قد يكون "قنبلة دماغية"

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كارثة صامتة.. "الهواء الملوث" يسرق نور عيون الأطفال

دراسة: رصد أول دليل على وجود "إشعاع هوكينغ"

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مواضيع ذات صلة

RNA Molecules are Extensively Processed Before They Become Functional

Phosphorylation Activates Pol II

Eukaryotic Promoters Are More Complex

Bacterial Promoters Are Relatively Simple

RNA Synthesis is A Cyclical Process that Involves RNA Chain Initiation, Elongation, & Termination

RNA is Synthesized From A DNA Template by RNA Polymerases

DNA & Chromosome Integrity Is Monitored Throughout the Cell Cycle

All Organisms Contain Elaborate Evolutionarily Conserved Mechanisms to Repair Damaged DNA

Micro-RNA and Long Noncoding RNA

Histone Organization

Noncoding DNA

DNA Synthesis Occurs During the S Phase of the Cell Cycle