TFIID

TFIID (also known as transcription factor II D) is a multi-subunit general transcription factor involved in the formation of the pre-initiation complex  TFIID binding is involved in the first step of transcription in eukaryotes. It later goes on to associate with RNA polymerase II, which is used to transcribe mRNA.

Structure

Using [electron microscopy] to study both human and [yeast] TFIID's had led scientists discover that both have a trilobular structure. TFIID is comprised of a TATA-binding protein (TBP) and a complex of 10 TBP-associated factors (TAFs). The TBP region is required by all three RNA polymerases, whereas the TAF regions differ by RNA polymerase . TBP is composed of 180 amino acid residues and is highly conserved throughout evolution .

Mechanism

TBP recognises and binds to the TATA box in eukaryotic genes as per it's main function . The TAF subunits are not required in genes with a TATA promoter, however they do have a role in reducing transcription levels by dissociating TBP from the TATA box .

Unlike other proteins with DNA binding domains, TBP notably alters the structure of DNA when bound via twisting and bending it . This creates a hydrophobic exposed region for TFIIB to join to in the next step of pre-initiation complex formation .

Function

While the main function of the TBP region of TFIID is to bind to the TATA box on DNA, the TAF proteins have a range of functions. Notably, TAFII250 acts as a scaffold protein and has kinase activity, and TAFII150 binds to DNA downstream of the TATA box . Furthermore, TAFII80, TAFII31 and TAFII20 have show similar features and structures to histones H4, H3 and H2b respectively .