Transposition of group II intron al1 in yeast and invasion of mitochondrial genes at new locations

Abstract

INTRON mobility at the RNA level^{1& ndash;8} by splicing reversal at allelic (homing) and non-allelic locations (transposition) has been reported in vitro^{9& ndash;12}. In the living cell, however, only intron homing by unidirectional gene conversion has been described^5,6,13,14. Supposing that intron insertions at non-allelic sites might occur in vivo, we speculated that group II splice-site-associated macro-deletions^{15& ndash;18} in fungal mitochondrial DNA might result from group II intron transposition to new locations followed by recombination. We used polymerase chain reaction techniques to detect this critical, infrequent intermediate in mtDNA populations. Here we report on group II intron al1 transposition to non-allelic, splicing-compatible locations within the cox1 gene of yeast mtDNA. The identified integration sites are preceded by motifs similar to the upstream exon A1. Sequences flanking intron al1 are not co-converted to the insertion sites and cis- and trans-acting mutations within al1 reduce intron mobility below detection levels. These findings suggest the involvement of an RNA intermediate in group II intron transposition.