Minimal catalytic domain of a group I self-splicing intron RNA

The self-splicing intron ribozymes have been regarded as primitive forms of the splicing machinery for eukaryotic pre-mRNAs. The splicing activity of group I self-splicing introns is dependent on an absolutely conserved and e xceptionally densely packed core region composed of two helical domains, P3 -P7 and P4-P6, that are connected rigidly via base triples. Here we show th at a mutant group I intron ribozyme lacking both the P4-P6 domain and the b ase triples can perform the phosphoester transfer reactions required for sp licing at both the 5' and 3' splice sites, demonstrating that the elements required for splicing are concentrated in the stacked helical P3-P7 domain. This finding establishes that the conserved core of the intron consists of two physically and functionally separable components, and we present a mod el showing the architecture of a prototype of this class of intron and the course of its molecular evolution.