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The transcription cycle. The transcription cycle can be described in six steps: (1) Template binding and closed RNA polymerase-promoter complex
formation: 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
Source: RNA Synthesis, Processing, & Modification, Harper's Illustrated Biochemistry, 30e
change and then is able to move away from the promoter, transcribing down the transcription unit. (5) Chain elongation: Successive residues are added to
Citation:
VW,RNA
Bender
DA, Botham
Kennellytermination
PJ, Weil P. signal
Harper's
Biochemistry,
30e;
2015 Available
at: release: Upon
the 3′-OH terminus
of Rodwell
the nascent
molecule
until a KM,
transcription
(T) Illustrated
is encountered.
(6) Chain
termination
and RNAP
http://mhmedical.com/
Accessed:
August
03,
2017
encountering the transcription termination site RNAP undergoes an additional conformational change that leads to release of the completed RNA chain,
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the DNA template
and RNAP.
can rebind
to DNAAllbeginning
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.