Structural insights into group II intron catalysis and branch-site selection

Group II self-splicing introns catalyze autoexcision from precursor RNA tran-scripts by a mechanism strikingly similar to that of the spliceosome, an RNA-protein assembly responsible for splicing together the protein-coding parts of most eukaryotic pre-mRNAs. Splicing in both cases initiates via nucleophilic attack at the 59 splice site by the 29 OH of a conserved intron adenosine residue, creating a branched (lariat) intermediate. Here, we describe the crystal struc-ture at 3.0 resolution of a 70-nucleotide RNA containing the catalytically essential domains 5 and 6 of the yeast ai5g group II self-splicing intron, revealing an unexpected two-nucleotide bulged structure around the branch-point adenosine in domain 6. Removal of intron RNA from coding se-quences is an essential step in the maturation of eukaryotic pre-messenger RNAs. Group II self-splicing introns, found in organellar genes of yeast, plants, and fungi and in eu