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12.7: Splicing of group II introns

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    1. Similar mechanism as that for nuclear pre‑mRNA splicing.
    2. Can occur by self‑splicing, albeit under rather artificial conditions.
    3. Reaction can be reversible (as can splicing of group I introns), leading to the idea that these introns can be transposable elements.
    4. The group II self‑splicing may be the evolutionary ancestor to nuclear pre‑mRNA splicing

    I. Mechanistic similarties for splicing group I, group II and pre‑mRNA introns

    1. All involve transesterification = phosphoester transfers. No high energy bonds are utilized in the splicing process; the arrangement of phosphodiester bonds is reorganized, and as a result exons are joined together.

    2. The initiating nucleophile is the 3' OH of a guanine nucleotide for Group I introns, whereas for Group II introns and introns in pre‑mRNA, it is the 2' OH of an internal adenine nucleotide in the intron.

    3. In all cases, particular secondary structures in the RNAs are utilized to bring together the reactive components (e.g. ends of exons and introns). These secondary structures may be intramolecular in the case of self‑splicing Group I and Group II introns, or they may be intermolecular in the case of pre‑mRNA and the snRNAs, e.g. those in the U1, U2, perhaps U6 snRNPs.

    Figure 3.3.18. Common features of the mechanism of splicing in Group I introns and in Group II introns plus introns in precursor to mRNA.