HOMING OF A GROUP-II INTRON IN YEAST MITOCHONDRIAL-DNA IS ACCOMPANIEDBY UNIDIRECTIONAL CO-CONVERSION OF UPSTREAM-LOCATED MARKERS

Group II introns ai1 and ai2 of the Saccharomyces cerevisiae mitochond rial COXI gene encode proteins having a dual function (maturase and re verse transcriptase) and are mobile genetic elements. By construction of adequate donor genomes, we demonstrate that each of them is self-su fficient and practises homing in the absence of homing-type endonuclea ses 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 specie s. 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 rec ipient mtDNAs. Importantly, studies of the behaviour of these introns in interspecific crosses demonstrate that flanking marker co-conversio n accompanying group II intron homing is unidirectional and efficient only in the 3' to 5' direction towards the upstream exon. Thus, the po lar co-conversion and dependence of the splicing proficiency of the in tron 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.