The mitochondrial genome of the fission yeast Schizosaccharomyces pombe. 2. Localization of genes by interspecific hybridization in strain ade7-50h- and cloning of the genome in small fragments.

A series of 18 small overlapping restriction fragments has been cloned, covering the complete mitochondrial genome of Schizosaccharomyces pombe. By hybridizing mitochondrial gene probes from Saccharomyces cerevisiae and Neurospora crassa with restriction fragments of Schizosaccharomyces pombe mitochondrial DNA, the following homologous genes were localized on the mitochondrial genome of S. pombe: cob, cox1, cox2 and cox3, ATPase subunit 6 and 9 genes, the large rRNA gene and both types of open reading frames occurring in mitochondrial introns of various ascomycetes. The region of the genome, hybridizing with cob exon probes is separated by an intervening sequence of about 2500 bp, which is homologous with the first two introns of the cox1 gene in Saccharomyces cerevisiae (class II introns according to Michel et al. 1982). Similarly, in the cox1 homologous region, which covers about 4000 bp, two regions were detected hybridizing with class I intron probes, suggesting the existence of two cox1 introns in Schizosaccharomyces pombe. Hybridization with several specific exon probes with a determined order has revealed that cob, cox1, cox3 and the large rRNA gene are all transcribed from the same DNA strand. The low intensities of hybridization signals suggest a large evolutionary distance between Schizosaccharomyces pombe and Saccharomyces cerevisiae or Neurospora crassa mitochondrial genes. Considering the length of the mitochondrial DNA of Schizosaccharomyces pombe (about 19.4 kbp) and the expected length of the localized genes and intron sequences there is enough space left for encoding the expected set of tRNAs and the small rRNA gene. The existence of leader-, trailer-, ori- and spacer sequences or further unassigned reading frames is then restricted to a total length of about 3000 bp only.