3po3
Arrested RNA Polymerase II reactivation intermediate
Structural highlights
FunctionRPB1_YEAST DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Largest and catalytic component of RNA polymerase II which synthesizes mRNA precursors and many functional non-coding RNAs. Forms the polymerase active center together with the second largest subunit. Pol II is the central component of the basal RNA polymerase II transcription machinery. During a transcription cycle, Pol II, general transcription factors and the Mediator complex assemble as the preinitiation complex (PIC) at the promoter. 11-15 base pairs of DNA surrounding the transcription start site are melted and the single stranded DNA template strand of the promoter is positioned deeply within the central active site cleft of Pol II to form the open complex. After synthesis of about 30 bases of RNA, Pol II releases its contacts with the core promoter and the rest of the transcription machinery (promoter clearance) and enters the stage of transcription elongation in which it moves on the template as the transcript elongates. Pol II appears to oscillate between inactive and active conformations at each step of nucleotide addition. Elongation is influenced by the phosphorylation status of the C-terminal domain (CTD) of Pol II largest subunit (RPB1), which serves as a platform for assembly of factors that regulate transcription initiation, elongation, termination and mRNA processing. Pol II is composed of mobile elements that move relative to each other. The core element with the central large cleft comprises RPB3, RBP10, RPB11, RPB12 and regions of RPB1 and RPB2 forming the active center. The clamp element (portions of RPB1, RPB2 and RPB3) is connected to the core through a set of flexible switches and moves to open and close the cleft. A bridging helix emanates from RPB1 and crosses the cleft near the catalytic site and is thought to promote translocation of Pol II by acting as a ratchet that moves the RNA-DNA hybrid through the active site by switching from straight to bent conformations at each step of nucleotide addition. In elongating Pol II, the lid loop (RPB1) appears to act as a wedge to drive apart the DNA and RNA strands at the upstream end of the transcription bubble and guide the RNA strand toward the RNA exit groove located near the base of the largely unstructured CTD domain of RPB1. The rudder loop (RPB1) interacts with single stranded DNA after separation from the RNA strand, likely preventing reassociation with the exiting RNA. The cleft is surrounded by jaws: an upper jaw formed by portions of RBP1, RPB2 and RPB9, and a lower jaw, formed by RPB5 and portions of RBP1. The jaws are thought to grab the incoming DNA template, mainly by RPB5 direct contacts to DNA. Publication Abstract from PubMedDuring gene transcription, RNA polymerase (Pol) II moves forwards along DNA and synthesizes messenger RNA. However, at certain DNA sequences, Pol II moves backwards, and such backtracking can arrest transcription. Arrested Pol II is reactivated by transcription factor IIS (TFIIS), which induces RNA cleavage that is required for cell viability. Pol II arrest and reactivation are involved in transcription through nucleosomes and in promoter-proximal gene regulation. Here we present X-ray structures at 3.3 A resolution of an arrested Saccharomyces cerevisiae Pol II complex with DNA and RNA, and of a reactivation intermediate that additionally contains TFIIS. In the arrested complex, eight nucleotides of backtracked RNA bind a conserved 'backtrack site' in the Pol II pore and funnel, trapping the active centre trigger loop and inhibiting mRNA elongation. In the reactivation intermediate, TFIIS locks the trigger loop away from backtracked RNA, displaces RNA from the backtrack site, and complements the polymerase active site with a basic and two acidic residues that may catalyse proton transfers during RNA cleavage. The active site is demarcated from the backtrack site by a 'gating tyrosine' residue that probably delimits backtracking. These results establish the structural basis of Pol II backtracking, arrest and reactivation, and provide a framework for analysing gene regulation during transcription elongation. Structural basis of RNA polymerase II backtracking, arrest and reactivation.,Cheung AC, Cramer P Nature. 2011 Mar 10;471(7337):249-53. Epub 2011 Feb 23. PMID:21346759[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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