Presentation 7(9-7-11)

Presentation 7(9-7-11) - Presentation 7 "Chapter 21 !...

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1 RNA Splicing and Processing Topics The Spliceosome Assembly pathway An Alternative Spliceosome uses different snRNP’s to Process the minor class of introns Pre-mRNA Splicing likely shares the mechanism with Autocatalytic group II Introns Splicing is temporally and functionally coupled with multiple steps in gene expression Alternative Splicing is a rule, rather than an exception in mutlicellualr eukaryotes Splicing can be regulated by Exonic and Intronic splicing enhancers and silencer Presentation 7 Chapter 21 Fas receptor Note: 2 additional questions were added to Problem set 7
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Splicing Mechanism Overview: Before splicing occurs, the spliceosome must identify the splice sites between introns and exons. Key sequences: 5’ and 3’ splice sites branch point nucleotide (located within the intron) polypyrimidine tract (located before the 3’ splice site) TransesteriFcation reaction: the 2’ OH of an adenosine residue (located within the intron itself) attacks the exon 1-intron boundary, detaching the intron from the exon producing a branched intron strucuture (lariat) next, the terminal 3’ OH of the newly released exon, attacks the intron-exon 2 junction splicing together the two exons and releasing the lariat intron. snRNAs perform many of the splicesome’s mRNA recognition events: form Watson-Crick base pairs with the precursor mRNA and one another. 5’ splice site is recognized by U1 snRNP other splice site consensus sequences are recognized by non-snRNP factors the branch point is recognized by the branch-point-binding protein (BBP) the polypyrimidine tract and 3’ splice site are bound by U2A± (U2 auxiliary factor) = U2A±65 and U2A±35 Molecular details of the individual steps are not fully understood. However, many helicases promote and regulate RNA-based conformational rearrangements. It is also thought that the snRNAs themselves (U2 and U6 snRNAs are the prime candidates) must play a central role in catalysis.
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Commitment of Pre-mRNA to the Splicing Pathway U1 snRNP initiates splicing by binding to the 5 ! splice site by means of an RNA–RNA pairing reaction. o U1 snRNP also stimulates polyadenylation of a downstream 3' end-processing site, is a target of the apoptotic pathway, and is a potent autoantigen in systemic autoimmune diseases such as systemic lupus erythematosus. The commitment complex (or E complex) contains U1 snRNP bound at the 5 ! splice site and the protein U2AF bound to a pyrimidine tract between the branch site and the 3 ! splice site. SR proteins : a family of splicing factors with RNA binding and protein-protein interaction motifs. Bind to the pre-mRNA where they are thought to help recruit other components of the spliceosome to the 5’ and 3’ sites. Some SR proteins play an important role in catalysis.
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•The direct way of forming an E complex is for: –U1 snRNP to bind at the 5 ! splice site –U2AF to bind at a pyrimidine tract between the branch site and the 3
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Presentation 7(9-7-11) - Presentation 7 "Chapter 21 !...

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