Model Answer
0 min readIntroduction
Transcription, the process of copying DNA into RNA, is a fundamental step in gene expression. In eukaryotes, this process is significantly more complex than in prokaryotes, requiring the coordinated action of numerous proteins. Unlike prokaryotes where RNA polymerase directly binds to the promoter, eukaryotic transcription initiation necessitates the assembly of a pre-initiation complex (PIC) involving several general transcription factors (GTFs) and RNA polymerase II. These proteins work in a sequential and highly regulated manner to ensure accurate and efficient gene transcription.
The Requirement of Proteins for Eukaryotic Transcription Initiation
Eukaryotic transcription initiation is a multi-step process orchestrated by a complex interplay of proteins. It can be broadly divided into the following stages, each requiring specific protein factors:
1. Recognition and Binding to the Promoter Region
- TFIID: This is the first GTF to bind to the promoter region, specifically recognizing the TATA box (a DNA sequence rich in thymine and adenine) using its TATA-binding protein (TBP) subunit. TBP induces a bend in the DNA, marking the initiation site.
- TAFs (TBP-associated factors): These proteins associate with TBP and help in recognizing other promoter elements beyond the TATA box.
2. Assembly of the Pre-Initiation Complex (PIC)
Following TFIID binding, other GTFs are recruited in a specific order:
- TFIIA: Stabilizes the TFIID-DNA complex.
- TFIIB: Binds to TFIID and recruits RNA polymerase II. It also helps in positioning RNA polymerase II at the start site.
- TFIIF: Binds to RNA polymerase II and stabilizes its interaction with TFIIB. It also plays a role in promoter clearance.
- TFIIE: Recruits TFIIH to the PIC.
- TFIIH: Possesses helicase activity, unwinding the DNA double helix at the start site, and kinase activity, phosphorylating the C-terminal domain (CTD) of RNA polymerase II. This phosphorylation is crucial for promoter clearance and the transition to elongation.
3. Role of RNA Polymerase II and Mediator Complex
- RNA Polymerase II: The enzyme responsible for synthesizing mRNA. It is recruited to the promoter by TFIIB and TFIIF.
- Mediator Complex: A large multi-protein complex that acts as a bridge between the GTFs and RNA polymerase II, and between the polymerase and regulatory proteins bound to enhancers. It facilitates the assembly of the PIC and regulates transcription rates.
4. Promoter Clearance and Elongation
Once RNA polymerase II is phosphorylated by TFIIH, it can clear the promoter and begin elongation. The CTD of RNA polymerase II serves as a platform for recruiting RNA processing factors, such as capping enzymes, splicing factors, and polyadenylation factors.
| Transcription Factor | Function |
|---|---|
| TFIID | Binds to TATA box; initiates PIC assembly |
| TFIIB | Recruits RNA polymerase II |
| TFIIH | DNA helicase & kinase activity; phosphorylates RNA Pol II |
| RNA Polymerase II | Synthesizes mRNA |
| Mediator Complex | Bridge between GTFs, RNA Pol II & regulatory proteins |
Conclusion
In conclusion, eukaryotic transcription initiation is a highly regulated process requiring the coordinated action of numerous proteins, including general transcription factors, RNA polymerase II, and the mediator complex. The sequential assembly of these proteins at the promoter region, coupled with DNA unwinding and RNA polymerase II phosphorylation, ensures accurate and efficient gene expression. Disruptions in these protein interactions can lead to various developmental and disease states, highlighting the importance of understanding these molecular mechanisms.
Answer Length
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