Complex of proteins necessary for gene transcription in eukaryotes and archaea
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 (Pol 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.
Preinitiation complexes are also formed during RNA Polymerase I and RNA Polymerase III transcription.
Assembly (RNA Polymerase II).
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.
In Archaea.
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.
RNA Polymerase I (Pol I).
Formation of the Pol I preinitiation complex requires the binding of selective factor 1 (SL1 or TIF-IB) to the core element of the rDNA promoter. SL1 is a complex composed of TBP and at least three TBP-associated factors (TAFs). For basal levels of transcription, only SL1 and the initiation-competent form of Pol I (Pol Iβ), characterized by RRN3 binding, are required.
For activated transcription levels, UBTF (UBF) is also required. UBTF binds as a dimer to both the upstream control element (UCE) and core element of the rDNA promoter, bending the DNA to form an enhanceosome. SL1 has been found to stabilize the binding of UBTF to the rDNA promoter.
The subunits of the Pol I PIC differ between organisms.
RNA Polymerase III (Pol III).
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.
References.
 ;templatestyles src="Reflist/styles.css" / ;