RNA Polymerase II

spinqualitylabelsall models Yeast RNA Polymerase II complex with RNA (PDB code 1i6h) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Introduction RNAP II transcription process. RNA polymerase II (RNAP II) is an enzyme that transcribes protein-encoding genes, and it therefore is responsible for the synthesis of mRNA. There are three RNA polymerase enzymes found in eukaryotic nuclei but RNAP II is the most studied. RNAP II is a 550 kDa multi-protein complex that includes 12 subunits. Several transcription factors are used to bind promoters upstream of the start site and are necessary for joining RNAP II and DNA. Bound RNAP II transcribes DNA into a strand of messenger RNA. Messenger RNA (mRNA) is a single stranded RNA molecule that is complementary to the template strand of DNA. The mRNA stand transports genetic information from DNA to the ribosome, where it is used to specify the amino acid sequence for the production of proteins. Structural Components This section will briefly discuss the chief structural components involved in the mechanism. The clamp (magenta), wall (navy blue), rudder (red), bridge (orange), RNA (light blue), and DNA (blue) are depicted. See below for PDB's and residue numbers. To begin, the clamp swings to trap the DNA in the cleft. Further along, the wall sends the DNA template through the cleft in approximately a 90° turn. Both the clamp and wall are parts of the Rpb2 subunit. Further along in the process, the rudder separates the newly synthesized RNA strand from the DNA template. The DNA reforms into a double helix as it leaves RNA pol II. Other components of RNA pol II include the following: The jaw is the opening through which DNA enters. The funnel is what the NTP’s travel through to be incorporated into the growing RNA strand, and the pore is the end of the funnel. The bridge is an Rpb1 segment that translocates the DNA-RNA complex at the end of each cycle of catalysis. Magnesium is located within the active site and functions as the catalyst. Transcription Transcription can largely be divided into three sections: initiation, elongation, and termination. In the process of initiation, RNAP II recruits several general transcription factors (GTFs) to bind to the promoter region of the DNA, and this eventually forms the preinitiation complex (PIC). The DNA enters RNAP II through the clamp, and then it is unwound, creating a transcription bubble. With the DNA unwound, the active site of RNAP II catalyzes the synthesis of the first few RNA bonds. Once the carboxy-terminal domain (CTD) becomes phosphorylated, the clamp undergoes a conformation change to effectively trap the DNA, and a few of the GTFs dissociate, which changes complex to the Elongator complex. After initiation, the process of elongation begins with the entry of NTPs. These NTPs largely enter through the funnel, and this tends to be a slow process because the funnel is only 12 Å in diameter, meaning that only one NTP can go through at a time. Once they have passed through the funnel, the NTPs enter the active site which contains an Mg ion that is bound by three aspartate residues at positions D739, D741, and D743. If the NTP is complementary to the DNA strand, it is loaded onto the insertion site next to the RNA and held in place by the bridge. While in this insertion site, the metal ion and aspartate residues catalyze the reaction that forms a phosphodiester bond between the 3’ end of the RNA and the 5’ end of the NTP. This step then leads into a translocation step. To make room for the next NTP, there is a Brownian ratchet mechanism in which the nearby trigger loop undergoes a conformation change that causes the bridge to move to the next transition site. During the bridge’s transition, the DNA-RNA hybrid is partially help in place by the -amanitin. Once the bridge is in the new initiation site, the trigger loop returns to its original conformation, allowing the process to begin again. In the termination stage, the CTD becomes dephosphorylated, which acts as a signal to dissociate the elongation complex. Once this signal is received, the new mRNA is released from RNP II through the rudder. Before being released, its 3’ end is polyadenylated. The DNA is brought back together at the other end of the transcription bubble, returning it to its original double-stranded form. Finally, RNAP II and the remaining GTFs dissociate from the DNA. α-Amanitin α-Amanitin is a bicyclic octapeptide that adheres tightly with RNAP II, which blocks the elongation steps. α-amanitin binds in the funnel and interacts with the bridge helix and adjacent Rpb1, but it does not inhibit the RNA pol II’s interaction with NTP. Instead, α-amanitin likely challenges the bridge’s conformational change that is necessary for the purposed RNAP translocation step. α-Amanitin, found in the poisonous mushroom death cap, leads to death after several days. This time frame aligns with the rate at which mRNA’s and proteins turnover. The chemical structure of α-amanitin. General Transcription Factors In both eukaryotes and prokaryotes, the basic mechanism for initiating transcription is the same: protein factors selectively bind to promoter regions on DNA. Prokaryotes use sigma factors while eukaryotes use a complex of 6 GTFs. These GTFs are all named similarly and begin with TF, for transcription factor, followed by the Roman numeral II since they are involved in transcription by RNAP II. The combination of all the transcription factors bound to the DNA promoter region, in complex with RNAP II, is called the PIC. The formation of the PIC occurs in an ordered pathway, beginning with the TATA box which is a promoter region on DNA at position -27. Process of PIC formation: 1. TFIID contains a subunit named the TATA-binding protein (TBP), which recognizes and binds to the TATA box on the DNA promoter. 2. TFIIB and TFIIA and interact with TBP and are recruted to the promoter. 3. TFIIF binds directly to RNAP II and escorts it to the promoter while TFIIB helps the complex bind correctly. 4. TFIIE and TFIIH are sequentually recruited which completes the PIC. Once the PIC is formed, RNAP II initiates RNA synthesis and produces a short transcript. When RNAP II becomes phosphorylated, it releases some of the GTFs from the complex and moves away from the promoter. TFIID stays bound to the promoter and can reinitiate transcription. The transcription factors are replaced by a new six-protein complex call the Elongator. TFIIF and TFIIH both remain associated with RNAP II during elongation.

References

Bushnell, D. A.; Westover, K. D.; Davis, R. E.; Kornberg, R. D. Structural Basis of Transcription: An RNA Polymerase II-TFIIB Cocrystal at 4.5 Angstroms. Science. 2004, 303, 983-988

Brueckner, F. and Cramer, P. Structural Basis of Transcription Inhibition by -amanitin and Implications for RNA Polymerase II Translocation. Nature Structure and Molecular Biology. 2008, 15, 811-818.

Cramer, P.; Bushnell, D. A.; Kornberg, R. D. Structural Basis of Transcription: RNA Polymerase II at 2.8 Ångstrom Resolution. Science. 2001, 292, 1863-1876

Evans, D. A.; Fitch, D. M.; Smith, T. E.; Cee, V. J. Application of Complex Aldol Reactions to the Total Synthesis of Phorboxazole B. J. Am. Chem. Soc. 2000, 122, 10033-10046.

Gnatt, A. L.; Cramer, P; Fu, J.; Bushnell, D. A.; and Kornberg, R. D. Structural Basis of Transcription: An RNA Polymerase II Elongation Complex at 3.3 Å Resolution. Science. 2001, 292, 1876-1882 1i6h

Hahn, S. Structure and Mechanism of the RNA Polymerase II Transcription Machinery. Nature Structure and Molecular Biology. 2004, 11, 394-403.

He, Yuan, et al. Near-atomic resolution visualization of human transcription promoter opening. Nature 533.7603. 2016.

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Orphanides, George, Thierry Lagrange, and Danny Reinberg. The general transcription factors of RNA polymerase II. Genes & development 10.21. 1996. 2657-2683

Shah, N. et. al. Tyrosine-1 of RNA Polymerase II CTD Controls Global Termination of Gene Transcription in Mammals. Molecular Cell. 2018, 69, 48-61.

Uzman, A.; Voet, D. Student companion Fundamentals of biochemistry: life at the molecular level, 4th ed., Donald Voet, Judith G. Voet, Charlotte W. Pratt; John Wiley & amp; Sons, 2012.

Xu, J.; Lahiri, I.; Wang, W.; Wier, A.; Cianfrocco, M. A.; Chong, J.; Hare, A. A.; Dervan, P. B.; DiMaio, F.; Leschziner, A. E.; Wang, D. Structural Basis for the Initiation of Eukaryotic Transcription-coupled DNA Repair. Nature. 2017. 551, 653-657 5vvr

Xin, L.; Bushnell, D. A.; and Kornburg, R. D. RNA Polymerase II Transcription: Structure and Mechanism. Biochemica et Biophysica Acta. 2013, 1829, 2-8.

Yan, C., Dodd, T., He, Y., Tainer, J. A., Tsutakawa, S. E., & Ivanov, I. (2019). Transcription preinitiation complex structure and dynamics provide insight into genetic diseases. Nature Structural and Molecular Biology, 26(6), 397-406.

Alpha-aminitin chemical structure image courtesy of https://en.wikipedia.org/wiki/Alpha-Amanitin#/media/File:Alpha-amanitin_structure.png

Notes

From structural components:

Structural overview: [PDB: 5VVR: with highlighted sections mentioned below]

Bridge: Depicted: [PDB: 1I6H: 810-845.a]

Wall: Depicted: [PDB: 1R5U: 853-919.b; 933-972.b]

Clamp: Depicted: [PDB: 1R5U: 3-345.a; 1395-1435.a; 1158-1124.b]

Rudder: Depicted: [PDB: 5VVR: 306-321.a]

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This page was created as a final project for the Advanced Biochemistry course at Wabash College during the Fall of 2019. This page was reviewed by Dr. Wally Novak of Wabash College.

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