1. The transcription cycle
The transcription cycle. Transcription can be described in six steps: (1) Template binding and closed RNA polymerase-promoter complex formation: RNA polymerase (RNAP) binds to DNA and then locates a promoter (P), (2) Open promoter complex formation: once bound to the promoter, RNAP melts the two DNA strands to form an open promoter complex; this complex is also referred to as the preinitiation complex or PIC. Strand separation allows the polymerase to access the coding information in the template strand of DNA (3) Chain initiation: using the coding information of the template RNAP catalyzes the coupling of the first base (often a purine) to the second, template-directed ribonucleoside triphosphate to form a dinucleotide (in this example forming the dinucleotide 5′ pppApNOH 3′). (4) Promoter clearance: after RNA chain length reaches ~10–20 nt, the polymerase undergoes a conformational change and then is able to move away from the promoter, transcribing down the transcription unit. (5) Chain elongation: Successive residues are added to the 3′-OH terminus of the nascent RNA molecule until a transcription termination signal (T) is encountered. (6) Chain termination and RNAP release: Upon encountering the transcription termination site RNAP undergoes an additional conformational change that leads to release of the completed RNA chain, the DNA template and RNAP. RNAP can rebind to DNA beginning the promoter search process and the cycle is repeated. Note that all of the steps in the transcription cycle are facilitated by additional proteins, and indeed are often subjected to regulation by positive and/or negative-acting factors.
Source: Chapter 36. RNA Synthesis, Processing, & Modification, Harper's Illustrated Biochemistry, 29e
Citation: Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil P. Harper's Illustrated Biochemistry, 29e; 2012 Available at: Accessed: October 05, 2017
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