The general transcription factor TFIIB is a single polypeptide of approximately 35 kDa. There is a Zn-binding domain near the N terminus of TFIIB, and the C-terminal domain encompasses two imperfect repeats; between the N and C termini is a phylogenetically conserved region. The C terminus interacts with TBP and RNA Polymerase II, whereas the N terminus interacts with factor TFIIF and RNA polymerase II. TFIIB is a sequence-specific factor, and it interacts with the BRE element within the promoter.
TFIIB interacts with the Rpb1 subunit of RNA polymerase II to define transcription strat sites. Several activators directly bind TFIIB, and stimulate transcription. The N-terminus and the C-terminus can participate in intramolecular interactions, and this can be disrupted by specific activators by causing a conformational change in TFIIB.
TFIIA also binds the preinitiation complex along with TFIIB. However, TFIIA is not required for accurate initiation, but rather functions as a coactivator of transcription.