▲ FIGURE 12-35 Splicing mechanisms in group I and group II self-splicing introns and spliceosome-catalyzed splicing of pre-mRNA. The Intron Is shown In gray, the exons to be joined In red. In group I Introns, a guanosine cofactor (G) that Is not part of the RNA chaIn assocIates wIth the actIve sIte. The 3'-hydroxyl group of thIs guanosIne partIcIpates In a transesterIfIcatIon reactIon wIth the phosphate at the 5' end of the Intron; thIs reaction Is analogous to that Involving the 2'-hydroxyl groups of the branch-site A In group II Introns and pre-mRNA Introns spliced In splIceosomes (see FIgure 12-7). The subsequent transesterIfIcatIon that links the 5' and 3' exons Is similar In all three splicing mechanisms. Note that spliced-out group I Introns are linear structures, unlike the branched Intron products In the other two cases. [Adapted from P A. Sharp, 1987, Science 235:769.]

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