Preinitiation complex

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 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) .

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