Mobile Group II Introns as Ancestral Eukaryotic Elements

The duality of group II introns, capable of carrying out both self-splicing and retromobility reactions, is hypothesized to have played a profound role in the evolution of eukaryotes. These introns likely provided the framework for the emergence of eukaryotic retroelements, spliceosomal introns and other key components of the spliceosome. Group II introns are found in all three domains of life and are therefore considered to be exceptionally successful mobile genetic elements. Initially identified in organellar genomes, group II introns are found in bacteria, chloroplasts, and mitochondria of plants and fungi, but not in nuclear genomes. Although there is no doubt that prokaryotic and organellar group II introns are evolutionary related, there are remarkable differences in survival strategies between them. Furthermore, an evolutionary relationship of group II introns to eukaryotic retroelements, including telomeres, and spliceosomes is unmistakable. Mobile group II introns are both ribozymes and retroelements. They are believed to be the progenitors of spliceosomal introns and some components of the spliceosome and are also closely related to eukaryotic retroelements. Whereas prokaryotic group II introns act predominantly as mobile elements, organellar group II introns exhibit the features of domestication. Studying both provides clues into their evolutionary dynamics. Group II intron ribonucleoproteins also bear similarities to retrotransposons, telomerases, and viral polymerases.