No abstract

Group II introns ai1 and ai2 of the Saccharomyces cerevisiae mitochondrial COXI gene encode proteins having a dual function (maturase and reverse transcriptase) and are mobile genetic elements. By construction of adequate donor genomes, we demonstrate that each of them is self‐sufficient and practises homing in the absence of homing‐type endonucleases encoded by either group I introns or the ENS2 gene. Each of the S. cerevisiae group II self‐mobile introns was tested for its ability to invade mitochondrial DNA (mtDNA) from two related Saccharomyces species. Surprisingly, only ai2 was observed to integrate into both genomes. The non‐mobility of ai1 was clearly correlated with some polymorphic changes occurring in sequences flanking its insertion sites in the recipient mtDNAs. Importantly, studies of the behaviour of these introns in interspecific crosses demonstrate that flanking marker co‐conversion accompanying group II intron homing is unidirectional and efficient only in the 3′ to 5′ direction towards the upstream exon. Thus, the polar co‐conversion and dependence of the splicing proficiency of the intron reported previously by us are hallmarks of group II intron homing, which significantly distinguish it from the strictly DNA‐based group I intron homing and strictly RNA‐based group II intron transposition.

Two homologous group I introns, the second intron of the cyt b gene, from related Saccharomyces species differ in their mobility. The S.capensis intron is mobile and encodes the I‐ScaI endonuclease promoting intron homing, whilst the homologous S.cerevisiae intron is not mobile, but functions as an RNA maturase promoting splicing. These two intron‐encoded proteins differ by only four amino acid substitutions. Taking advantage of the remarkable similarity of the two intron open reading frames and using biolistic transformation of mitochondria, we show that the replacement of only two non‐adjacent residues in the S.cerevisiae maturase carboxy‐terminal sequence is sufficient to induce a homing‐endonuclease activity without losing the splicing function. Also, we demonstrate that these two activities reside in the S.capensis bi2‐encoded protein which functions in both splicing and intron mobility in the wild‐type cells. These results provide new insight into our understanding of the activity and the evolution of group I intron‐encoded proteins.