Preinitiation complex

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A preinitiation complex (abbreviated PIC) is an ordered combination of six general transcription factors:

The preinitiation complex combines with RNA polymerase II near the transcriptional start site, to support a basal level of transcription.

For transcription of a region of DNA to occur an enzyme called RNA Polymerase II is required (RNA Pol I is involved in ribosomal RNA gene transcription, whilst RNA Pol III is involved in tRNA gene transcription). This must bind to the TATA box, which is found at -30 (note the gene starts at the CAP site which is found at +1). In order for binding to occur efficiently transcription factors (TFs) are used. These have two domains - a DNA binding domain and an activation domain (which can either be acidic, glutamine rich or proline rich).

Transcription factors are called: TF II A, B, C, D, E, F, H and I. TF II D binds to the TATA box directly. TF II D is a complex of TATA binding protein (TBP - recognises and binds with high affinity) and TAFs (TBP associated factors - 8 are currently know with Mr between 18 and 250 KDa). TF II A then binds, followed by TF II B. TF II B recruits RNA Pol II, which always binds with TF II F. TF II E and TF II H then bind. The former is an ATPase which is required to provide energy, and the latter is a helicase, which allows unwinding of the DNA, which is necessary for transcription to occur. This is the preinitiation complex (PIC).

Around 75% of genes (estimated from human genome project) do not actually contain TATA boxes, instead they have an initiator element (Inr), which is a pyrimidine rich sequence located around -10 and is recognised by TF II I (contrary to what the other person has stated above), to which TF II D, and the rest of the PIC then binds.

This information has been taken from the lecture notes given to 3rd year biochemistry students at Southampton University during a Genes and Genetic Diseases lecture. The content is not copyrighted but care should be taken before copying as different people will say different things about the PIC and mistakes may have been made during the lecture. But as an overview on this topic this is accurate.