The mitochondrial genome of the fission yeast Schizosaccharomyces pombe: The cytochrome b gene has an intron closely related to the first two introns in the Saccharomyces cerevisiae cox1 gene

Abstract

The DNA sequence of the cob region of the Schizosaccharomyces pombe mitochondrial DNA has been determined. The cytochrome b structural gene is interrupted by an intron of 2526 base-pairs, which has an open reading frame of 2421 base-pairs in phase with the upstream exon. The position of the intron differs from those found in the cob genes of Saccharomyces cerevisiae, Aspergillus nidulans or Neurospora crassa. The Sch. pombe cob intron has the potential of assuming an RNA secondary structure almost identical to that proposed for the first two cox1 introns (group II) in S. cerevisiae and the p1- cox1 intron in Podospora anserina. It has most of the consensus nucleotides in the central core structure described for this group of introns and its comparison with other group II introns allows the identification of an additional conserved nucleotide stretch. A comparison of the predicted protein sequences of group II intronic coding regions reveals three highly conserved blocks showing pairwise amino acid identities of 34 to 53%. These regions comprise over 50% of the coding length of the intron but do not include the 5′ region, which has strong secondary structural features. In addition to the potential intron folding, long helical structures involving repetitive sequences can be formed in the flanking cob exon regions. A comparison of the Sch. pombe cytochrome b sequence with those available from other organisms indicates that Sch. pombe is evolutionarily distant from both budding yeasts and filamentous fungi. As was seen for the Sch. pombe cox1 gene (Lang, 1984), the cob exons are translated using the universal genetic code and this distinguishes Sch. pombe mitochondria from all other fungal and animal mitochondrial systems.