The 2'-OH group at the group II intron terminus acts as a proton shuttle

Group II introns are self-splicing ribozymes that excise themselves from precursor RNAs and catalyze the joining of flanking exons. Excised introns can behave as parasitic RNA molecules: they can catalyze their own insertion into DNA and RNA via a reverse splicing reaction. Previous studies have identified mechanistic roles for various functional groups located in the catalytic core of the intron and within target molecules. Here we introduce a new method for synthesizing long RNA molecules with a modified nucleotide at the 3' terminus. This modification allows us to examine the mechanistic role of functional groups adjacent to the reaction nucleophile. During reverse splicing, the 3'-OH group of the intron terminus attacks the phosphodiester linkage of spliced exon sequences. Here we show that the adjacent 2'-OH group on the intron terminus plays an essential role in activating the nucleophile by stripping away a proton from the 3'-OH and then shuttling it from the active site.