Transcription initiation is one of the key regulatory steps in the control of gene expression: initiating transcription at the correct promoter at the correct time is essential for executing the correct biological process. The transcription of eukaryotic genes requires one of three RNA polymerases: Pol I, Pol II, or Pol III. However, none of these RNA polymerases can initiate transcription on its own, instead requiring the aid of different sets of transcription factors. The TATA-box binding protein (TBP) is required for the accurate initiation of transcription by all three transcription factors from promoters with or without a TATA-box, associating with a variety of different transcription factors.
TBP has been explored most thoroughly for its association with Pol II, which is required for the transcription of protein-encoding genes into mRNA. Pol II associates with a host of other factors, including the general transcription factors TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH, to form huge multi-subunit pre-initiation complexes on the core DNA promoters (TATA box) at the mRNA start sites. To form these initiation complexes, TBP associates with distinct sets of TBP-associated factors (TAF), which enable the accurate initiation of transcription using specific RNA polymerases. For example, Pol II transcription can involve TFIID in which TBP associates with 13 to 14 different TAFs, or B-TFIID in which TBP associates with BTAF1. These associations can be highly specific. For example, the association of TBP with an unprocessed form of TFIIA in the TBP-TFIIA-containing complex (TAC) is specific for undifferentiated stem cells.