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9 Cards in this Set
- Front
- Back
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RNA Capping
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-Post-transcriptional (i.e., G not encoded)
-Involves adding a 7MeGuanosine nt to the first (RNA) nt in an unusual way, and often methylation of the first few nt of the RNA -Occurs before the pre-mRNA is 30 nt long -5'to 5' |
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Capping order of events
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1.RNA triphosphatase cleaves terminal phosphate.
2.Guanylyl transferase adds GMP blocking 5' end of RNA 3.Methyl transferase adds methyl group to terminal end 4.another methyl transferase adds another methyl group internally to cap1 site -AdoMet is the methyl donor |
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Cap Functions
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-Protection from some ribonucleases
-enhanced translation -enhanced transport to nucleus -enhanced splicing of first intron for some pre-mRNA's |
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Pre-mRNA polyadenylation
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-Most cytoplasmic mRNA's have a polyA tail (3'end)of 50-250 adenylates
-added post transcriptionally by an enzyme, polyA polymerases -turns over(recycles) in cytoplasm |
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Functions of the PolyA-tail
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1.promotes mRNA stability
-shortening of polya tail can trigger rapid degradation of mRNA 2.enhances translation -promotes recruitment of ribosomes -bound by a polya binding protein in the cytoplasm,PAB1 -synergistic stimulation with cap! |
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polyadenylation (polA) signal
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-AAUAAA in mammals and plants
-located 20-30bp from polyA tail -Mutagenesis and vivo studies show GU rich and U stretches downstream of AAUAAA can be important |
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Polyadenylation: the proteins for cleavage
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1.CPSF binds the AAUAAA
2.CstF binds the GU rich region cooperatively with CPSF 3.CFI and CFII are RNA binding proteins 4.PAP (polyA polymerase) 5.nRNAPII - stimulates cleavage-one unit unlike others which are complexes |
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polyadenylation mechanism (2 phases)
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phase 1: requires AAUAAA and 8nt downstream (3')
phase 2: once 10 A's are added, further adenylation does not require the AAUAAA. |
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Proteins required for polyadenylation
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phase 1: CPSF and PolyA polymerase (PAP II)
phase 2: PAPII and polyA binding protein (PABII) -PABII binds to short A-tail and helps PAPII synthesize long tails |
