Physical mapping of the mitochondrial DNA of the wild-type Saccharomyces cerevisiae strain RXII revealed that most of the restriction sites as well as the location of the apocytochrome b gene were identical in comparison with the known maps of the mitochondrial genome in other Saccharomyces cerevisiae strains. In the middle of the SalI linearized map of the RXII mitochondrial DNA, a deletion was detected which resulted in the loss of two EcoRI and one BamHI restriction sites. The corresponding region, however, exists in most other laboratory strains of Saccharomyces mapped so far. This region overlaps the introns aI2 and aI3 surrounding exon A3 sequences of the subunit 1 of the cytochrome oxidase gene. The nucleotide sequence of the subunit 1 gene showed that the BamHI site was located close to the aI3-A4 intron-exon junction and the distal EcoRI site close to the aI2-A2 boundary. I therefore conclude that these two introns are deleted in the mitochondrial genome of strain RXII. The exon A3 must have been conserved since this strain was respiratory competent. This result, while being a good example of the morphological diversity of a genome with the same function, may contribute to an understanding of the role of introns in the mitochondrial split genes in yeast.

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Mink M

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