Transposition and exon shuffling by group II intron RNA molecules in pieces

In the realms of RNA, transposable elements created by self-inserting introns recombine novel combinations of exon sequences in the background of replicating molecules. Although intermolecular RNA recombination is a wide-spread phenomenon reported for a variety of RNA-containing viruses, direct evidence to support the theory that modern splicing systems, together with the exon-intron structure, have evolved from the ability of RNA to recombine, is lacking. Here, we used an in vitro deletion-complementation assay to demonstrate trans-activation of forward and reverse self-splicing of a fragmented derivative of the group II intron bI1 from yeast mitochondria. We provide direct evidence for the functional interchangeability of analogous but non-identical domain 1 RNA molecules of group II introns that result in trans-activation of intron transposition and RNA-based exon shuffling. The data extend theories on intron evolution and raise the intriguing possibility that naturally fragmented group III and spliceosomal introns themselves can create transposons, permitting rapid evolution of protein-coding sequences by splicing reactions.