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lecture10 - RNA Processing and Export 1001 The 5 Cap(m7Gppp...

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1001 RNA Processing and Export
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1002 5'-->5' linkage The 5' Cap (m 7 Gppp): Structure and function Several enzymatic reactions add the m7Gppp cap to pre-mRNA : 1. Step 1: γ -phosphate removal from 5 '-end of RNA 2. Step 2: Transfer of GM P from GTP to the 5 ' diphosphate. 3. Step 3: Transfer of methyl groups from S-adenosylmethionine to the N7 position and the 2 ' -oxygens of ribose (In animals and plants, not yeast) (In vertebrates) Functions of the cap: 1. Splicing 2. RNA stability 3. Export 4. Translation Enzyme involved: RNA triphosphatase, capping enzyme, guanine-7-methyl transfrase, 2 -O- methyltransferase
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1003 AAAAA (100-250) Polyadenylation of mRNA 1 2 3 4 1 2 3 4 Factors co-transcriptionally bind two sequences in the RNA: the upstream AAUAAA signal the downstream GU- or U-rich sequence Poly(A) polymerase binds and facilitates cleavage at the poly(A) site. Bound poly(A) polymerase, in a slow phase, adds about 12 A s to the 3' end of the RNA. Poly(A)-binding protein accelerates poly(A) addition by poly(A) polymerase. Functions of polyadenylation : 1. Protect 3'-ends of RNAs 2. Alternative processing generates different messages 3. RNA export 4. Translation (see p. 131 fig. 4-31)
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1004 TBP mRNA sequence
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1005 mRNA Splicing maturing the message diversifying the message
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1006
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1007 exons introns 3’ UTR 5’ UTR cap poly(A) tail promoter ORF eukaryotic class II gene structure
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1008 mRNA Splicing and the Human Genome (~2.9 x 10 9 bp) [ ] Introns (~30%) Exons (~1.2%) Avg. exon length 145 nt Avg. intron length 3365 nt Avg. of 10-12 introns/gene
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1009 Splice Site Consensus Sequences Yeast Mammalian 5' Splice site 3' Splice site Branch point Exon 1 Exon 2 GUAUGU UACUAAC PyAG Exon 1 Exon 2 GU A G AGN NNCUNAC PyAG Exon 1 Exon 2 Splicing
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1010 Splice sites sequences are conserved Three conserved sequence elements are essential for splicing The 5 -splice site (ss), the 3 -splice site (ss) and the branch point site (BPS) 20-50 nts upstream of the 3 ss a conserved adenine surrounded by a 11 nt polypyrimidine tract serves as a branch point; The entire conserved sequence is known as the BPS
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1011 The splicing reaction An intron is removed in a
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