Sketch out a basic transcription unit and illustrate the RNA polymerase II associated assembly of a transcription initiation complex during expression of a eukaryotic gene. (IAS 2022/20 Marks)

Basic Transcription Unit

• Transcription Unit: The region of DNA that is transcribed into RNA. It includes:
  
  • Promoter: A region upstream of the gene that signals where transcription begins.
  • Coding Sequence: The part of the gene that is transcribed into RNA and later translated into protein.
  • Terminator: Signals the end of transcription.

Components of the Transcription Unit

• Promoter Region:
  
  • Found upstream of the coding sequence.
  • Contains consensus sequences like the TATA box (commonly found at position -30) which is essential for the binding of RNA polymerase II and initiation factors.
  • Includes other elements such as the CAAT box and GC box which also influence transcription initiation.
• Transcription Factors:
  
  • General Transcription Factors (GTFs): These are essential for initiating transcription. Key factors include:
    
    • TFIID: Contains the TATA-binding protein (TBP) which binds to the TATA box.
    • TFIIB: Binds to the DNA and helps recruit RNA polymerase II.
    • TFIIE, TFIIH, TFIIF: Aid in the assembly of the RNA polymerase II complex and play roles in promoter melting, and phosphorylation of RNA polymerase II.
• RNA Polymerase II:
  
  • The enzyme responsible for transcribing mRNA in eukaryotes.
  • It binds to the promoter region with the help of transcription factors and begins the synthesis of RNA.
• Enhancers and Silencers:
  
  • Enhancers are DNA sequences that increase the rate of transcription by binding activators.
  • Silencers are DNA sequences that decrease the rate of transcription by binding repressors.

RNA Polymerase II Associated Assembly of the Transcription Initiation Complex

• Binding of Transcription Factors:
  
  • The process begins when transcription factors, including TFIID, bind to the TATA box within the promoter region.
  • This facilitates the recruitment of other factors, including TFIIB and TFIIF, which help stabilize the RNA polymerase II binding.
• Recruitment of RNA Polymerase II:
  
  • RNA polymerase II is recruited to the promoter region with the help of TFIIF.
  • The binding of TFIIE and TFIIH is crucial for opening up the DNA and allowing the polymerase to start transcription.
• Formation of the Pre-Initiation Complex (PIC):
  
  • The combination of RNA polymerase II and transcription factors form the pre-initiation complex (PIC).
  • TFIIH has helicase activity that unwinds the DNA at the transcription start site and also has kinase activity that phosphorylates the C-terminal domain (CTD) of RNA polymerase II.
• Phosphorylation of RNA Polymerase II:
  
  • The phosphorylation of the CTD of RNA polymerase II is a key step in transitioning from transcription initiation to elongation.
  • This phosphorylation activates RNA polymerase II and allows it to begin RNA synthesis.
• Transcription Initiation:
  
  • RNA polymerase II starts synthesizing RNA in the 5’ to 3’ direction, copying the template strand of DNA.
  • The transcription initiation complex ensures the accurate start of transcription at the correct location.

Conclusion

The assembly of a transcription initiation complex during the expression of a eukaryotic gene involves the recruitment of transcription factors and RNA polymerase II to the promoter region of the gene. This complex process ensures that the genetic information stored in DNA is accurately transcribed into RNA, allowing for the synthesis of proteins essential for cellular function.