Preinitiation complex

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The preinitiation complex (PIC) is a complex of proteins that is formed in eukaryotic cells previous initiation of transcription. It consists of RNA polymerase II and a series of transcription factors (TF), which have different functions and are RNA polymerase specific. The main function of the preinitiation complex is to recruit RNA polymerase, that is necessary to undergo transcription, and to allow binding of this enzyme with the basal promoter region of DNA.

There are six types of transcription factors that participate in the formation of the PIC: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH.TFIID is the first one to bind to the TATA box basal promoter region of DNA and consists of a TATA binding protein (TBP) subunit and TBP-associated factors (TAFs)[1].

TFIIB is the transcription factor that recruits RNA pol II while TFIIH has an helicase activity and unwinds the DNA strand before transcription starts.

Forming the Pol II Transcription PIC

The order of assembly of the pre-initiation complex is deduced by in vitro experiments. Therefore, when this complex forms in a cell (in vivo), additional factors may be required. In order for transcription to occur RNA polymerase II needs to be recruited. The six transcription factors mentioned above are all involved in recruiting the enzyme and some are also involved in unwinding the double-stranded DNA during transcription.

  1. TFIID binds to the TATA box on the basal promoter region.
  2. The binding of TFIID to the TATA is facilitated by TFIIA. TFIIA facilitates the binding by interacting with the central subunit of TFIID (TBP - TATA box binding protein) to form a complex.
  3. After the complex formation, TFIIB is recruited and this interacts with TFIID by also interacting with the TBP subunit of TFIID.
  4. The addition of TFIIB is the allows the recruitment for RNA Pol II to the complex. RNA Pol II comes together with TFIIF to the complex.
  5. The preinitiation complex is completed with the addition of TFIIE and TFIIH. TFIIE binds to the complex first. The binding creates a site for TFIIH dock onto and completing the formation of the complex[2].
  6. Hydrolysis action occurs as TFIIH helicase activity separates the template strands.
  7. As the promoter melts, TFIIA and TFIID stay behind while TFIIB, TFIIE and TFIIH are released. TFIIF moves down the template along with PolII

A youtube video highlights how this pre-initiation complex forms around the TATA box and how this initiates the melting of DNA to start transcription in Eukaryotes - https://www.youtube.com/watch?v=XedDRqbbIOg

References

  1. B.Turner(2001), Chromatin and gene regulation: machanisms in epigenetics (1st ed), Oxford: Blackwell Science
  2. Harvey Lodish, Arnold Berk, Chris A. Kaiser, Monty Krieger, Matthew P. Scot, Anthony Bretscher, Hidde Ploegh, Paul Matsudaira (2008). Molecular Cell Biology. 6th ed. New York: W. H. Freeman and Company.

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