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: TFA, TFB, TFD, TFE, TFF, and TFH. Additional regulatory complexes (such as the mediator coactivator and chromatin remodeling complexes) may also be components of the PIC. Assembly. 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). Other preinitiation complexes. Archaea have a preinitiation complex resembling that of a minimized Pol II PIC, with a TBP and an Archaeal transcription factor B (TFB, a TFIIB homolog). The assembly follows a similar sequence, starting with TBP binding to the promoter. An interesting aspect is  that the entire complex is bound in an inverse orientation compared to those found in eukaryotic PIC. They also use TFE, a TFIIE homolog, which assists in transcription initiation but is not required. Pol I initiation start with UBTF (UBF) recognizing an upstream control element (UCE) located around ~100 to 200 bp upstream. It recruits Selective factor 1 (TIF-IB), which is a complex of TBP and three units of TBP-associated factor. UBF then recognizes the core control elements. Phosphorylated RRN3 (TIF-IB) binds Pol I. The entire complex recognizes UBF/SL1, binds to it, and starts transcribing. The  precise usage of subunits differ among organisms. Pol III has three classes of initiation, which start with different factors recognizing different control elements but all converging on TFIIIB (similar to TFIIB-TBP; consists of TBP/TRF, a TFIIB-related factor, and a B″ unit) recruiting the Pol III preinitiation complex. The overall architecture resembles that of Pol II. Only TFIIIB needs to remain attached during elongation.