Preinitiation complex

The preinitiation complex (abbreviated PIC) is a large complex of proteins that is necessary for the transcription of protein-coding genes in eukaryotes (+archaea). The preinitiation complex helps position RNA polymerase II over gene transcription start sites, denatures the DNA, and positions the DNA in the RNA polymerase II active site for transcription.cite journal | author = Lee TI, Young RA | title = Transcription of eukaryotic protein-coding genes | journal = Annu. Rev. Genet. | volume = 34 | issue = | pages = 77–137 | year = 2000 | pmid = 11092823 | doi = 10.1146/annurev.genet.34.1.77]

Typically the PIC is made up of six general transcription factors: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH.

In 2007 Roger D. Kornberg proposed the following model of the PIC at promoters with TATA boxes:cite journal | author = Kornberg RD | title = The molecular basis of eukaryotic transcription | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 104 | issue = 32 | pages = 12955–61 | year = 2007 | pmid = 17670940 | doi = 10.1073/pnas.0704138104]

* TATA binding protein (TBP, a subunit of TFIID) binds the promoter, creating a sharp bend in the DNA. This bend wraps the DNA around RNA polymerase II and the C-terminal domain of TFIIB. TBP binds the TATA box, and the spacing of the TATA box relative to the transcription start site (25-30 base pairs upstream of the transcription start site) ensures that RNA polymerase II will be properly positioned.
* The N-terminal domain of TFIIB brings the DNA into proper position for entry into the active site of RNA polymerase II.
* TFIIE joins the growing complex and recruits TFIIH.
* Subunits within TFIIH that have ATPase and helicase activity create negative superhelical tension in the DNA.
* Negative superhelical tension causes the DNA to denature and form the transcription bubble. TFIIF binds to the coding strand of the single-stranded, denatured DNA and keeps the bubble open.
* The single-stranded non-coding strand of the DNA can then bend and enter the RNA polymerase II active site.
* If transcription proceeds more than six bases, TFIIB is displaced and RNA polymerase II escapes the promoter region to transcribe the remainder of the gene.

External links

*Descriptive images
** [http://www.bmb.psu.edu/nixon/bmb400/Image5.gifbmb.psu.edu]
** [http://www.imcb.osaka-u.ac.jp/en/seminar/image/04_img1.jpgimcb.osaka-u.ac.jp]
** [http://www.biochem.ucl.ac.uk/bsm/xtal/teach/trans/pol2.gifbiochem.ucl.ac.uk]

References