TRANSPOSABLE GROUP-II INTRONS IN FISSION AND BUDDING YEAST - SITE-SPECIFIC GENOMIC INSTABILITIES AND FORMATION OF GROUP-II IVS PLDNAS

The recent report on RNA-mediated group II intron (IVS, intervening se quence) transposition in mitochondria (mt) of Saccharomyces cerevisiae and Podospora anserina and the demonstration of reverse transcriptase (RT) activity encoded by the mobile S. cerevisiae intron cox1-aI1 sug gests that group II introns constitute a new class of site-specific re tro-like (retroid) elements. This is supported by the finding that the mitochondrial cob1-bI1 intron from the fission yeast Schizosaccharomy ces pombe, encoding an RT-like open reading frame, is transposed in mt DNA populations. In agreement with the involvement of an RNA-intermedi ate in IVS transposition: First, the insertion sites were preceded by at least an IBS1-like (intron binding site) motif, which corresponds t o the upstream exon and suffices to form the IBS1/EBS1 (EBS: exon bind ing site) base-pairing interactions. Second, intron transposition was conservative with respect to sequences flanking the insertion sites. W e formulated the hypothesis that transient IVS insertion at non-alleli c sites followed by recombination can be viewed as a general molecular mechanism, applicable equally well to site-specific genomic instablil ities involving splice-site borders of group II introns and to the for mation of extra-genomic IVS plasmid DNAs (plDNAs). We used polymerase chain reaction (PCR) techniques to detect infrequent rearrangements in mtDNA and report here on duplicative IVS transposition, twintron form ation (e.g. bI1 insertion into another bI1 intron) and IVS insertions at canonical 5' exon-intron borders in S. pombe (cob1 -bI1) and in S. cerevisiae (cox1-aI1). These data substantiate the concept that group II intron homing; IVS transposition and circular IVS plDNA formation i nvolve a common RNA-mediated mechanism. Finally: the findings suggest that extra-genomic group II IVS copies are not restricted to senescenc e mycelia of P. anserina, but constitute natural components of group I I IVS-containing genomes.