【正文】 SITE FEEDBACK CONTACT TELL A FRIEND Last modified May 10, 2022 The Official Web Site of The Nobel Foundation Copyright169。 rRNA), phage ? Eukaryotes – lower (algae, protists, amp。339。 II. PAP: poly(A) polymerase. PABPII: poly(A)binding protein II. Poly(A) tail stabilizes mRNA and enhances translation and export into the cytoplasm. RNA is cleaved 10~35nt 3’ to A2UA3. The binding of PAP prior to cleavage ensures that the free 3’ end generated is rapidly polyadenylated. PAP adds the first 12A residues to 3’OH slowly. Binding of PABPII to the initial short poly(A) tail accelerates polyadenylation by PAP. The polyadenylation plex is associated with the CTD of Pol II following initiation. Functions of 5’ cap and 3’ polyA ? Need 5’ cap for efficient translation: – Eukaryotic translation initiation factor 4 (eIF4) recognizes and binds to the cap as part of initiation. ? Both cap and polyA contribute to stability of mRNA: – Most mRNAs without a cap or polyA are degraded rapidly. – Shortening of the polyA tail and decapping are part of one pathway for RNA degradation in yeast. mRNA Halflife ? t189。 CFII: cleavage factor I amp。 2022 The Nobel Foundation Background: Adenovirus has a DNA genome and makes many mRNAs. Can we determine which part of the genome encodes for each mRNA by making a DNA:RNA hybrid? Experiment: Isolate Adenovirus genomic DNA, isolate one adenovirus mRNA, hybridize and then look by EM at where the RNA hybridizes (binds) to the genomic DNA. Surprise: The RNA is generated from 4 different regions of the DNA! How can we explain this? Splicing!! The discovery of split genes (1977) 1993 Noble Prize in Medicine To Dr. Richard Robert and Dr. Phillip Sharp The matured mRNAs are much shorter than the DNA templates. DNA mRNA Exon and Intron ? Exon is any segment of an interrupted gene that is represented in the mature RNA product. ? Intron is a segment of DNA that is transcribed, but removed from within the transcript by splicing together the sequences (exons) on either side of it. Exons are similar in size Introns are highly variable in size GTAG rule ? GTAG rule describes the presence of these constant dinucleotides at the first two and last two positions of introns of nuclear genes. ? Splice sites are the sequences immediately surrounding the exonintron boundaries ? Splicing junctions are recognized only in the correct pairwise binations The sequence of steps in the production of mature eukaryotic mRNA as shown for the chicken ovalbumin gene. The consensus sequence at the exon–intron junctions of vertebrate premRNAs. 4 major types of introns 4 classes of introns can be distinguished on the basis of their mechanism of splicing and/or characterisitic sequences: – Group I introns in fungal mitochondria, plastids, and in prerRNA in Tetrahymena (selfsplicing) – Group II introns in fungal mitochondria and plastids (selfsplicing) – Introns in premRNA (spliceosome mediated) – Introns in pretRNA Group I and II introns The sequence of transesterification reactions that splice together the exons of eukaryotic premRNAs. Splicing of Group I and II introns ? Introns in fungal mitochondria, plastids, Tetrahymena prerRNA ? Group I – Selfsplicing – Initiate splicing with a G nucleotide – Uses a phosphoester transfer mechanism – Does not require ATP hydrolysis. ? Group II – selfsplicing – Initiate sp