Transcription preinitiation complex

The preinitiation complex (abbreviated PIC) is a complex of approximately 100 proteins that is necessary for the transcription of protein-coding genes in eukaryotes and archaea. The preinitiation complex positions RNA polymerase II at gene transcription start sites, denatures the DNA, and positions the DNA in the RNA polymerase II active site for transcription.

The minimal PIC includes RNA polymerase II and six general transcription factors: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH. Additional regulatory complexes (such as the mediator coactivator and chromatin remodeling complexes) may also be components of the PIC.

A classical view of PIC formation at the promoter involves the following steps:

An alternative hypothesis of PIC assembly postulates the recruitment of a pre-assembled "RNA Polymerase II holoenzyme" directly to the promoter (composed of all, or nearly all GTFs and RNA polymerase II and regulatory complexes), in a manner similar to the bacterial RNA polymerase (RNAP).

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