Pre Initiation Complex

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In eukaryotic gene expression the Pre Initiation Complex (PIC) mediates the binding between the genome's promoter and RNA polymerase. Each type of RNA polymerase has a different PIC[1]. Assembly of the PIC[2] is required for initiating transcription. General transcription factors and RNA polymerase join together at the promoter, forming the PIC[3].

Subunits

The complexity of PIC varies between the types of eukaryotic RNA polymerase. For polymerases I and III the complex consists of only a few subunits.

For polymerase II the PIC is far more complex. It consists of 6 General Transcription Factors (GTF). TFIID is the core recognition protein. It is made up of TATA binding protein (TBP) and TBP associated factor (TAF). TBP binds to the TATA box, initiating the assembly of PIC. TFIIA and TFIIB will then join. TFIIA stabilizes the binding complex while TFIIB helps recruit RNA polymerase II. TFIIB interacts directly with RNA polymerase II which has TFIIF attached to it. This allows TFIIE and finally TFIIH to bind. TFIIH has helicase activity and starts seperating the strands at the start site, allowing the formation of an open complex. This process is known as promoter melting and it requires ATP-hydrolysis. TFIIH also contains one of the kinases that phosphorylates the C-terminal domain (CTD) of RNA polymerase II. Phosphrylation of CTD is essential for the transition from intiation to elongation. When all GTFs and RNA polymerase II are recruited at the promoter, the assembly of PIC is complete.

However, initiating the assembly of PIC on a TATA-less promoter (promoter without TATA box but other core promoter elements) requires TAFs which responds to the activators. 

References

  1. Cassimeris,L. et al., 2007; p219 Lewin's cells. 2nd ed. Sudbury: Jones and Bartlett.
  2. Sainsburys S, Bernecky C, Cramer P. Structural basis of transcription initiation by RNA polymerase II. Nature Review Molecular Cell Biology. 2015; 16:129-43.
  3. Lodish, H. (2016). Molecular cell biology. New York : W.H. Freeman Macmillan Learning. 2016.
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