Initiation of Transcription by RNA Polymerase
To begin transcription, eucaryotic RNA polymerase II requires the general transcription factors.
These transcription factors are called TFIIA, TFIIB, and so on. (A) The promoter contains a DNA sequence called the TATA box, which is located 25 nucleotides away from the site where transcription is initiated. (B) The TATA box is recognized and bound by transcription factor TFIID, which then enables the adjacent binding of TFIIB. (C) For simplicity the DNA distortion produced by the binding of TFIID is not shown. (D) The rest of the general transcription factors as well as the RNA polymerase itself assemble at the promoter. (E) TFIIH uses ATP to pry apart the double helix at the transcription start point, allowing transcription to begin. TFIIH also phosphorylates RNA polymerase II, releasing it from the general factors so it can begin the elongation phase of transcription. As shown, the site of phosphorylation is a long polypeptide tail that extends from the polymerase molecule.
The exact order in which the general transcription factors assemble on each promoter is not known with certainty. In some cases, most of the general factors are thought to first assemble with the polymerase independent of the DNA, with this whole assembly then binding to the DNA in a single step. The general transcription factors have been highly conserved in evolution; some of those from human cells can be replaced in biochemical experiments by the corresponding factors from simple yeasts.
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