The general transcription factors were named TFIIA, B, D, E, F, I, H, J. Each transcription factor was isolated and purified. Next, transcription rates upon the addition of the factor to reactions containing RNA polymerase II and other subcellular fractions were monitored (Latchman 1995). TFIID binds the TATA box and is facilitated by TFIIA, or TFII I for systems without TATA boxes (Pederson 1994, Latchman 1995, Carey 2000). TFIIB, RNA polymerase II, TFIIF and TFIIH join the TFIID-TFIIA-DNA complex in a stepwise order. TFIIE may assist in this assembly stage (Pederson 1994, Latchman 1995, Carey 2000). This is known as the basal transcription complex (Carey 2000). TFIIJ completes the formation of the complex (Pederson 1994). TFIID is a multi-protein complex that contains the TATA-binding protein (TBP) and other proteins called TBP-associated factors (TAFIIs)( Pederson 1994, Latchman 1995, Carey 2000).
Experiments including Dnase I footprinting and chemical crosslinking have determined that when the TBP dramatically bends the DNA when it binds and wraps the promoter around TFIID. The carboxyl terminal domain of the largest subunit of RNA polymerase II is phosphorylated by TFIIH, which converts RNA polymerase II into elongation mode (polymerase IIo) (Pederson 1994, Latchman 1995, Sentenac 1990). TFIIH also phosphorylates TFIID, TFIIE and TFIIF (Latchman 1995). It has been suggested that this transcription factor melts the DNA to form the open complex that allows TBP to bind (Carey 2000). In addition, it plays a critical role in DNA excision repair, forming a link between transcription and DNA repair (Carey 2000, Latchman 1995). Once elongation is initiated, TFIID and TFIIB remain at the start site to catalyze another initiation complex while TFIIF remains associated with the RNA elongation complex (Pederson 1994, Latchman 1995). An alternative initiation system involves RNA polymerase already associated with TFIIB, TFIIF and TFIIH without DNA. Thus, after TFIID and TFIIA have bound DNA, the complex of RNA polymerase II and the transcription factors bind reducing the number of steps for complex formation (Pederson 1994).