CSB349L14 - CSB349 Lecture 14 RNA splicing November 3rd...

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CSB349 Lecture 14 November 3 rd RNA splicing Regulation of the Message How gene expression is regulated at the level of RNA. Prior to this we’ve discussed rate of transcription, control of transcription by chromatin changes by HATs, control transcription by direct changes to DNA like transcription. Beyond this point, you can dictate where the RNA is localized, where and if it is used as a protein, control it’s stability, etc. At this level, there are a number of mechanisms such as RNA splicing. When A gene is being transcribed, RNA gets capped at 5’ and there are other modifications that happen. RNA splicing occurs wuickly as primary transcript gets transcribed. Transcription, RNA shuttling, RNA processing, RNA splicing. Machinery for splicing is loaded on before transcript is finished. Important part is to remove introns. (exons in grey). Introns in purple are removed by RNA splicing.
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CSB349 Lecture 14 November 3 rd The GU-AG introns are the ones we will discuss. Not all are processed in the same way though. BY far, in most genes in most multicellular euk, the intron sequences account for the vast majority of the gene despite that they are removed and discarded. There’s quite a bit of info in introns. Some are processed into non- coding DNAs. Table on bottom is an example of how much percentage can be taken up by introns.
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CSB349 Lecture 14 November 3 rd The important feature of RNA splicing is recognizing which part should be spliced, cut and pasted. Sequence information is critical for this process, there are proteins as well as RNA which will recognize specific sites along the transcript to be spliced. Top 2 are GU and bottom 2 are AG. The 5’ site is the donor splice site and the 3’ is the acceptor splice site. There is also the A (branch point) that is present somewhere in middle of intron, critical component of actual splicing mechanism. There is also a poly Py tract near 3’ splice site, which is important for regulating splicing.
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CSB349 Lecture 14 November 3 rd The biochemistry of splicing was worked out in the 1990s. There are two steps in this chemical reaction. There are 2 Transesterification rxns. This is where the branch point A is important, it serves as the donor for electrons by its OH group to PO4 gropu, which will convert Exon 1 end to OH moiety. The OH acts as second nuc, attacking PO4 at 5’ end of the second exon, a bond occurs b/w the two exons and there’s a circularized form created. NO proteins involved! The intrinsic chemical properties of the RNA sequence is sufficient for driving the biochemical rxn. The proteins involved here are incredibly important. ? The intron will later get degraded.
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CSB349 Lecture 14 November 3 rd You do need proteins for where and how splicing takes place, just not for actual splicing. There are genes with multiple exons and multiple introns. In the cases where all introns of genes are spliced, you need to control where splicing takes place such that the branch points don’t control other intron splicing. If this problem is NOT avoided, you get skipping of exons. Exon skipping can be useful or detrimental.
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