Transcription 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.^[1]

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

Assembly

In 2007 Roger D. Kornberg proposed the following model of the PIC at promoters with TATA boxes:^[2]

• 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
  • bmb.psu.edu
  • imcb.osaka-u.ac.jp
  • biochem.ucl.ac.uk

References