General transcription factor
General transcription factors (GTFs), also known as basal transcriptional factors, are a class of protein transcription factors that bind to specific sites (promoter) on DNA to activate transcription of genetic information from DNA to messenger RNA. GTFs, RNA polymerase, and the mediator multiple protein complex constitute the basic transcriptional apparatus that bind to the promoter, then start transcription. GTFs are also intimately involved in the process of gene regulation, and most are required for life.
A transcription factor is a protein that binds to specific DNA sequences (enhancer or promoter), alone or with other proteins in a complex, to control the rate of transcription of genetic information from DNA to messenger RNA by promoting as an activator or blocking as a repressor the recruitment of RNA polymerase. As a class of protein, general transcription factors bind to promoters along the DNA sequence or form a large transcription preinitiation complex to activate transcription. General transcription factors are necessary for transcription to occur.
- 1 Types
- 2 Function and Mechanism
- 3 Research Examples about GTFs
- 4 References
- 5 External links
In archaea and eukaryotes, transcription initiation requires an RNA polymerase and a set of multiple GTFs to form a transcription preinitiation complex. The Transcription initiation by eukaryotic RNA polymerase II involves the following GTFs:
Function and Mechanism
In bacteria-Sigma factor (σ factor)
A sigma factor is a protein needed only for initiation of RNA synthesis in bacteria. Sigma factors provide promoter recognition specificity to the RNA polymerase (RNAP) and contribute to DNA strand separation, then dissociating from the RNA polymerase core enzyme following transcription initiation. The RNA polymerase core associates with the sigma factor to form RNA polymerase holoenzyme. Sigma factor reduces the affinity of RNA polymerase for nonspecific DNA while increasing specificity for promoters, allowing transcription to initiate at correct sites. The core enzyme of RNA polymerase has five subunits (protein subunits) (~400 kDa). Because of the RNA polymerase association with sigma factor, the complete RNA polymerase therefore has 6 subunits: the sigma subunit-in addition to the two alpha (α), one beta (β), one beta prime (β'), and one omega (ω) subunits that make up the core enzyme(~450 kDa). In addition, many bacteria can have multiple alternative σ factors. The level and activity of the alternative σ factors are highly regulated and can vary depending on environmental or developmental signals.
In archaea and eukaryotes (Transcription preinitiation complex)
The transcription preinitiation complex is a large complex of proteins that is necessary for the transcription of protein-coding genes in eukaryotes and archaea. It attaches to the promoter of the DNA (e.i., TATA box) and helps position the RNA polymerase II to the gene transcription start sites, denatures the DNA, and then starts transcription.
Transcription preinitiation complex assembly
The assembly of transcription preinitiation complex follows these steps:
- TATA binding protein (TBP), a subunit of TFIID (the largest GTF) binds to the promoter (TATA box), creating a sharp bend in the promoter DNA. Then the TBP-TFIIA interactions recruit TFIIA to the promoter.
- TBP-TFIIB interactions recruit TFIIB to the promoter. RNA polymerase II and TFIIF assemble to form the Polymerase II complex. TFIIB helps the Pol II complex bind correctly.
- TFIIE and TFIIH then bind to the complex and form the transcription preinitiation complex. TFIIA/B/E/H leave once RNA elongation begins. TFIID will stay until elongation is finished.
- Subunits within TFIIH that have ATPase and helicase activity create negative superhelical tension in the DNA. This negative superhelical tension causes approximately one turn of DNA to unwind and form the transcription bubble.
- The template strand of the transcription bubble engages with the RNA polymerase II active site, then RNA synthesis starts.
Research Examples about GTFs
Interaction of the Human Androgen Receptor Transactivation Function with the General Transcription Factor TFIIF
This experiment attempted to understand how the human androgen receptor regulates gene transcription through interaction with GTF. The human androgen receptor (AR) is a ligand-activated transcription factor that regulates genes important for male sexual differentiation and development. In this experiment, a panel of general transcription factors was screened for interactions with the receptor transactivation domain. A polypeptide containing amino acids 142–485 of the human receptor was expressed and purified. This region was used because in a previous study, it was known that this region plays an important role in the full activity of human AR. Next, the purified protein was allowed to adsorb onto the surface of a microtiter plate and incubated with 35S-labeled General transcription factors TFIIB, TBP, TFIIE, TFIIF, and TFIIH. The result showed that this region of the Human Androgen Receptor N terminus containing the transactivation function bound selectively to the basal transcription factors TBP and TFIIF. This means that TBP and TFIIF play the most important role in the regulation of male sexual differentiation and development through their interaction with Human AR and its ligand.
The acetylation of histones can increase the accessibility of nucleosomal DNA to transcription factors. The characteristics of three histones acetyltransferases — Human GCN5 homolog PCAF (p300/CBP-associated factor), the transcription coactivator p300/CBP, and TAFII250 — had previously provided a potential explanation for the relationship between histone acetylation and transcriptional activation. Therefore, this experiment attempted to find if these acetyltransferases could acetylate directly to GTF to affect transcription. The recombinant transcription factors studied were TFIIA (p55 and p12 subunits), TFIIB, TATA-binding protein (TBP), the α and β subunits of TFIIE, TFIIF (RAP30 and RAP74 subunits), and the core histones H3 and H4. They first normalized the amount of each factor studied by Coomassie blue staining after SDS-polyacrylamide gel electrophoresis. Each transcription factor was first incubated with recombinant PCAF, p300, or TAFII250 in the presence of [3H]acetyl CoA. The result showed that the β subunit of TFIIE was acetylated by all three enzymes. Both subunits of TFIIF-RAP74 and RAP30 were acetylated by PCAF and p300, but TAFII250 had little effect on this factor. Therefore, the acetyltransferases can acetylate directly to GTF to affect transcription.
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- General Transcription Factors at the US National Library of Medicine Medical Subject Headings (MeSH)
- Holoenzymes at the US National Library of Medicine Medical Subject Headings
- Interaction of the Human Androgen Receptor Transactivation Function with the General Transcription Factor TFIIF
- Acetylation of General Transcription Factors by Histone Acetyltransferases
- DNA Transcription YouTube Video