RNA Processing.BCH6415.Spr2006.Part I print

RNA Processing.BCH6415.Spr2006.Part I print - RNA...

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RNA PROCESSING BCH 6415 Dr. Yang Spring 2006 Part I
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5’ Capping of mRNA RNA triphosphatase RNA guanylyl transferase RNA methyltransferase In mammals, single bifunctional capping enzyme
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Proudfoot et al. Cell 108:501; 2002 5’ mRNA capping in eukaryotes : Protects mRNA from ribonucleases: mRNA’s without caps rapidly degraded by 5’ 3’ exonuclease Directs mRNA for processing and transport to cytoplasm Regulates protein synthesis by promoting efficient initiation of translation Only pol II transcripts are capped probably due to association of capping and transcription machineries Yeast capping enzymes (guanylyltransferase & methyltransferase) are bound to CTD of pol II Capping enzyme binds only to phosphorylated form of CTD Capping of pre-mRNA occurs early in transcription: In Drosophila, majority of nascent mRNA’s 30 nt long already capped
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Shatkin & Manley. Nature Struc. Biol . 7:838; 2000 a, Pol II A form containing unphosphorylated CTD initiates transcription, produces 20–25 nucleotide 5' triphosphorylated transcripts, and pauses with SPT5 (see refs 18, 19) bound as part of a large transcription complex. b, CTD is phosphorylated in the O form, changes conformation and binds capping enzyme (CE). CE modifies the exposed end of the nascent transcript, stimulated by SPT5 as well as P-CTD. MT binds to CE (mammals) or P-CTD (yeast) and to the terminal GpppN (mammals). c, Imp stimulates MT substrate binding and N7 methylation of the cap G, and Pol II O form switches from initiation to processive elongation. SPT5 binds to unphos. form of pol II; interacts with capping enz.; released after CTD phosphorylation Capping enz. stimulated by phosphorylated CTD and SPT5 Importin-α; stimulates methyltransferase
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Varani. Structure 5:855; 1997 Multiple Roles of the 5’ Cap During Gene Expression in Eukaryotes . Red dot = 5’ mRNA cap CBC = Cap-binding complex Pab1p = cytoplasmic poly(A)-binding protein Role in pre-mRNA splicing: Cap-binding complex (CBC) interacts with m7-G cap Cap structure enhances formation of spliced mRNA For example, cap promotes efficient interaction between U1 snRNP and 5’splice site (early step in spliceosome assembly) How cap and splicing components interact currently unknown.
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