Retrotransposition of a yeast group II intron occurs by reverse splicing directly into ectopic DNA sites

Group II introns, the presumed ancestors of nuclear pre-mRNA introns, are s ite-specific retroelements. In addition to "homing" to unoccupied sites in intronless alleles, group II introns transpose at low frequency to ectopic sites that resemble the normal homing site. Two general mechanisms have bee n proposed for group II intron transposition, one involving reverse splicin g of the intron RNA directly into an ectopic DNA site, and the other involv ing reverse splicing into a site in RNA followed by reverse transcription a nd integration of the resulting cDNA by homologous recombination. Here, by using an "inverted-site" strategy, we show that the yeast mtDNA group II in tron all retrotransposes by reverse splicing directly into an ectopic DNA s ite. This same mechanism could account for other previously described ectop ic transposition events in fungi and bacteria and may have contributed to t he dispersal of group II introns into different genes.