Mechanism for Group II intron splicing



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  1. Self-splicing reaction

    Some Group II introns have the ability to splice themselves, i.e. no external energy and no proteins required, the RNA itself is catalytic ("ribozymes"). Self-splicing of group II introns occurs via a lariat intermediate and two trans-esterification reactions similar to the ones occurring for nuclear pre-mRNA.




    In the first step, a bulged adenosine in domain 6 attacks the 5' splice site, resulting in cleavage of the 5' exon and formation of a lariat intermediate.




    In the second step, the 5'and 3' exons are ligated together and the intron is released as a lariat.




    Note:
    1. Two different reactants are used (2'-hydroxyl, 3'-hydroxyl).
    2. Choice of splice sites is determined by group II conserved sequences.
    3. Proteins are often involved but not absolutely required.
    4. Lariat structure: Unusual 5'-2'-phosphodiester branch is cleaved after intron removal by a debranching enzyme.


  2. Protein-assisted splicing reaction

    Although some group II introns self-splice in vitro, splicing in vivo requires the additional assistance of protein. The most important protein in facilitating splicing in vivo is the intron-encoded RT.




    The protein binds to unspliced intron RNA at a high affinity binding site in domain 4A with secondary contacts in domains 1, 2 and 6. Together, the protein-RNA interactions result in conformational changes in the intron that result in self-splicing.




    After splicing, the RT remains tightly bound to spliced intron, and this RNP particle is the active moiety in subsequent mobility reactions.

    Last modified: Tue Feb 10 11:09:15 CET 2004