Figure 2.

Universal aspects in transcription initiation mechanisms. (A) Initiation in eubacteria requires the binding of a sigma factor to the RNA polymerase (Pol) to form the holoenzyme. This event induces a conformational change in the sigma factor, enabling it to recognize specific sequences of proximal promoter elements, leading to transcription initiation. Activators may facilitate this recognition step through interactions with holoenzyme components. (B) Initiation on class II promoters may involve analogous steps of holoenzyme assembly with GTFs binding to RNA polymerase II (Pol II), depicted here to emphasize parallels to the eubacterial paradigm. This holoenzyme (or RNA polymerase II and unbound GTFs) is able to recognize the initiation region via TFIID, but only when TFIID is bound to the core promoter DNA in a particular conformation(s) and not in another(s). The transcriptionally active conformation of TFIID can be induced by a large variety of upstream-bound activators via the coactivator function of certain TAFs and soluble cofactors.