Table 1.

Complexes Involved in RNApII PIC assembly

Protein Complex	Functions
RNApII	12 Subunits; catalyzes transcription of all mRNAs and a subset of noncoding RNAs including snoRNAs and miRNAs
TFIIA	2–3 subunits; functions to counteract repressive effects of negative cofactors like NC2; acts as a coactivator by interacting with activators and components of the basal initiation machinery
TFIIB	Single subunit; stabilizes TFIID-Promoter binding; aids in recruitment of TFIIF/Pol II to the promoter; directs accurate start site selection
TFIID	14 subunits including TBP and TBP Associated Factors (TAFs); nucleates PIC assembly either through TBP binding to TATA sequences or TAF binding to other promoter sequences; coactivator activity through direct interaction of TAFs and gene specific activators
TFIIE	2 subunits; helps recruit TFIIH to promoters; stimulates helicase and kinase activities of TFIIH; binds ssDNA and is essential for promoter melting
TFIIF	2–3 subunits; tightly associates with RNApII; enhances affinity of RNApII for TBP-TFIIB-promoter complex; necessary for recruitment of TFIIE/TFIIH to the PIC; aids in start site selection and promoter escape; enhances elongation efficiency
TFIIH	10 subunits; ATPase/helicase necessary for promoter opening and promoter clearance; helicase activity for transcription coupled DNA repair; kinase activity required for phosphorylation of RNApII CTD; facilitates transition from initiation to elongation
Mediator	At least 24 subunits; bridges interaction between activators and basal factors; stimulates both activator dependent and basal transcription; required for transcription from most RNApII dependent promoters
SAGA	20 subunits; interacts with activators, histone H3, and TBP; histone acetyltransferase activity; deubiquitinating activity
Trf1	TBP related factor identified in Drosophila; upregulated in CNS and gonads during development; can bind TATA sequences; mostly found at RNApIII dependent promoters as part of TFIIIB but also required at a subset of RNApII dependent promoters
Trf2	TBP related factor identified in all metazoans; cannot bind TATA sequences; important for histone gene expression in Drosophila
Trf3	TBP related factor identified in vertebrates; can bind TATA sequences; important for differentiation of muscle cells in mammals and for haematopoietic cell development in zebrafish.
TFIIS	1 subunit; stimulates intrinsic transcript cleavage activity of RNApII allowing backtracking to resume RNA synthesis after transcription arrest; stimulates PIC assembly at some promoters
NC2	2 subunits; binds TBP/DNA complexes and blocks PIC assembly; can have both positive and negative effects on transcription
Mot1/bTAF1	1 subunit; induces dissociation of TBP/DNA complexes in ATP dependent manner; can have both positive and negative effects on transcription