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.