The 19430 bp long mitochondrial (mt) DNA of the ascomycete S. pombe has been completely sequenced (1,2). This mt genome is very tightly packed with genes coding for 25 tRNAs, the two ribosomal RNA subunits, cytochrome oxidase subunits 1, 2 and 3 (cox1, cox2 and cox3), cytochrome b (cob), ATPase subunits 6, 8 and 9 and an unassigned open reading frame (orf227, previously designated as Urf a (2)). As in many other fungal mt DNAs, genes are coded on the same DNA strand and are separated by moderately A+T-rich 'non-coding' sequences up to 220 bp in length. Contrasting code deviations regularly found in mitochondria of other Ascomycetes, the universal translation code is used in all standard (assigned) protein- coding mt genes of S. pombe (few UGA codons are present in the flanking regions of intron orfs, the second cox1 intron and the cob intron, as well as in orf227. It remains unresolved if these UGA codons represent stops, are decoded (ineffeciently ?) as tryptophan, and if the respective proteins are translated at all.
Consequently, the standard S. pombe mt mRNAs can be efficiently and faithfully translated in vitro, in a reticulocyte translation system without addition of mt tRNAs. The cox1 gene contains two group I introns (3) and the cob gene one group II intron (4). All three introns include open reading frames characteristic of the corresponding intron group, i.e. two highly conserved dodecapeptide motifs in the group I intronic ORFs (cf. 5) and conserved reverse transcriptase sequence motifs in the group II intronic ORF (6). All three intronic ORFs are in phase with the upstream exon reading frames. The genome is transcribed in two units (indicated by arrows in Figure 1), starting from the two promoters situated opposite on the circular DNA molecule, upstream from the large subunit rRNA and cox3 genes. tRNA genes are interspersed between most other genes and are used for processing of the large RNA precursors. A similar mechanism has been found in animal mitochondria (c.f. 7).