fall 09 lecture 28

fall 09 lecture 28 - Current model of spliceosomal...

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1 Current model of spliceosomal splicing, cont. 1. U4/U6 (extensively base paired with each other) and U5 bind; tri- complex binds to 3’ intron splice site (again, due to complementarity between U5 and 3’ intron splice site); U4 is inhibitor of U6 1. U4 and U6 change conformation due to the binding. Assembly of the spliceosome (B1 complex) is complete. 1. U1 attaches to 5 splice site of intron (due to the complementarity between sequences) 1. U2 binds to branch point A (again, due to complementary sequences). This step requires ATP (A complex).
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2 1. U6 dissociates from U4, displacing U1 at the 5’ splice site (with blue exon); U1 and U4 are released. ATP-dependent step. 1. Once U1 and U4 are released, the splicesome is activated; it contains only U2, U5 and U6 snRNPs (B2 complex); U2-U6 represent the active site of the splicesome Current model of spliceosomal splicing, cont.
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3 1. U5 is responsible for positioning and holding; U2-U6 complex has catalytic activity (ribozyme): mediates splicing at both sites- very fast (formation of C1 and C2 complexes); 1. Second splicing step requires ATP 1. Lariat degraded, making splicing irreversible 1. snRNPs are probably released as individual particles and recycled Current model of spliceosomal splicing, cont.
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4 What is catalytically active component of snRNP? Pre-mRNA are spliced by same reactions that occur in self-splicing group II introns The snRNAs required for splicing are very similar to parts of the self- splicing group II introns; combined snRNAs ( U6 and U2 ) have similar structure (conformation) to the structure of self-splicing group II introns snRNA are catalytically active components of snRNPs (ribozyme) Proteins probably have supplementary role: sites of interaction with regulatory factors, maintaining of proper 3D structure, transport (similar to the role of maturases)…
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5 Control of gene expression through RNA processing and post- transcriptional / pre-translational control Learning Objectives Alternative splicing and trans-splicing Describe role of transport through nuclear pore in contro of gene expression Describe different mechanisms of post- transcriptional/pre-translational control of gene expression
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6 Transcription initiation •Direct influence of A and R on initiation Activators (A), repressors (R) and remodeling proteins and chromatin structure •Methylation •Hormones and other “external” signals RNA processing •Alternative splicing and trans-splicing polyadenylation Transport through the nuclear pore Posttranscriptional / pretranslational control (events on/with mRNA before ribosomes) •Localization of mRNA •RNA editing •Post-transcriptional silencing by siRNA or miRNA •Translational control switch •RNA stability (degradation and stabilization) Translational control (mRNA/ribosomes) •Phosphorylation of translation initiation factors •Upstream AUG codons •IRES Protein activity control •Posttranslational modification and transport Control of gene expression: overall
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fall 09 lecture 28 - Current model of spliceosomal...

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