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slides-15a - #15 RNA processing(I 1 2 3 4 RNA splicing 5...

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#15 RNA processing (I) 1. RNA splicing 2. 5' CAP addition 3. 3' polyadenylation 4. Transcription termination
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What happen to RNA during/after transcription? capping
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1. Eukaryotic RNAs are synthesized in the nucleus as the primary transcript (e.g. pre-mRNA), which will be processed in the nucleus and then transported to the cytosol. 2. The pre-mRNA is generally processed in 3 steps: Capping, intron-splicing, and polyadenylation . Three processes are catalyzed by different enzymes, but CTD of RNA polymerase II is involved in recruiting RNA processing enzymes. So some RNA processing steps occur before transcription is completed . 3. The 5'-cap and 3'-polyA are usually found only in mRNAs. Introns are found in other types of genes, including those code for rRNAs, and tRNAs. Similar mechanism is used to process introns of different genes. About eukaryotic RNA processing
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Major types of mRNA processing Splicing —removal of introns and joining of exrons Capping -addition of a special G ( m 7 G ) to the 5' end of mRNA Polyadenylation —addition of a poly(A) tail to the 3' end of mRNA
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Almost all eukaryotic mRNAs are spliced to remove introns. Spliceosome is required for splicing of most genes. But some RNA may catalize its own splicing (self-splicing) 1. RNA splicing 1. Introns varies in number and size (100 bp to 500 kb), animal genes usually have more and longer introns than plant genes 2. Exrons are often shorter than introns. The mature mRNA may contain as little as ~5% of the pre-mRNA sequences
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Two earlier experiments showing the existence of introns (1970's) genomic DNA cDNA 1. Comparison of genomic DNA sequence of a gene with its cDNA sequence shows many genomic sequences (introns) are missing in the cDNA. 2. When DNA of a gene is hybridized with its mRNA, and observed under electron- microscope, various loops were often observed. Those loops are introns that found no counterpart in mRNA to hybridize with.
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SS are consensus sequences within introns of many genes that are necessary for splicing ( 5’ GU------A---AG 3’ ): (1) 5’ of an intron is usually GU (2) 3’ of an intron is usually AG (3) branch-point sequence that always contain an A somewhere, but the exact sequence may vary in different genes or organisms Splicing signals (SS)
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the intron with exon 1 is broken, in exchange for a new ester bond between 5' end of the intron and the 2'-oxygen of the branch-site A located inside of the intron ( lariat ). 2. Second, the ester bond linking 3' of intron and 5' of exon 2 is broken, in exchange for the second new ester bond between exon 1 and exon 2. RNA splicing involves two sequential transesterification reactions Transesterification means transfer of an ester bond from one pair of polymers to another. exon 1
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This note was uploaded on 02/20/2009 for the course LS 3 taught by Professor Lin during the Spring '06 term at UCLA.

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slides-15a - #15 RNA processing(I 1 2 3 4 RNA splicing 5...

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