Group II self-splicing introns in bacteria

Abstract

LIKE nuclear premessenger introns, group II self-splicing introns are excised from primary transcripts as branched molecules, containing a 2′–5′ phosphodiester bond. For this reason, it is widely believed that the ribozyme (catalytic RNA) core of group II introns, or some evolutionarily related molecule, gave rise to the RNA components of the spliceosomal splicing machinery of the eukaryotic nucleus¹. One difficulty with this hypothesis has been the restricted distribution of group II introns. Unlike group I self-splicing introns, which interrupt not only organelle primary transcripts, but also some bacterial and nuclear genes^2–5, group II introns seemed to be confined to mitochondrial and chloroplast genomes (reviewed in ref. 6). We now report the discovery of group II introns both in cyanobacteria (the ancestors of chloroplasts⁷) and the γ subdivision of purple bacteria, or proteobacteria⁸, whose α subdivision probably gave rise to mitochondria⁹. At least one of these introns actually self-splices in vitro.