RNA polymerase II combines with the preinitiation complex (PIC) near the transcriptional start site, to support a basal level of transcription in type II genes.
Transcription inititation of all protein encoding genes in eukaryotes requires the formation of the preinitiation complex, which comprises RNA polymerase II and the (basal) general transcription initiation factors: TFIIA, TFIIB, TFIID/TATA-box binding protein (TBP), TFIIE, TFIIF, and TFIIH.
It is generally accepted that the preinitiation complex is nucleated through binding of the TATA-box binding protein (TBP) to the promoter, either through a step-wise assembly of other basal factors or through recruitment of a pre-assembled holoenzyme. TBP recognizes the TATA-box in both orientations, yet the PIC possesses directionality that may result from the direction specific TFIIBc−DNA interface (ref).
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