NUCLEOTIDE-SEQUENCE OF THE COX1 GENE IN KLUYVEROMYCES-LACTIS MITOCHONDRIAL-DNA - EVIDENCE FOR RECENT HORIZONTAL TRANSFER OF A GROUP-II INTRON

The cytochrome oxidase subunit 1 gene (COX1) in K. lactis K8 mtDNA spans 8 826 bp and contains five exons (termed E1-E5) totalling 1 602 bp that show 88% nucleotide base matching and 91% amino acid homology to the equivalent gene in S. cerevisiae. The four introns (termed Kl cox1.1-1.4) contain open reading frames encoding proteins of 786, 333, 319 and 395 amino acids respectively that potentially encode maturase enzymes. The first intron belongs to group II whereas the remaining three are group I type B. Introns Kl cox1.1, 1.3, and 1.4 are found at identical locations to introns Sc cox1.2, 1.5a, and 1.5b respectively from S. cerevisiae. Horizontal transfer of an intron between recent progenitors of K. lactis and S. cerevisiae is suggested by the observation that Kl cox1.1 and Sc cox1.2 show 96% base matching. Sequence comparisons between Kl cox1.3/Sc cox1.5a and Kl cox1.4/Sc cox1.5b suggest that these introns are likely to have been present in the ancestral COX1 gene of these yeasts. Intron Kl cox1.2 is not found in S. cerevisiae and appears at an unique location in K. lactis. A feature of the DNA sequences of the group I introns Kl cox1.2, 1.3, and 1.4 is the presence of 11 GC-rich clusters inserted into both coding and non-coding regions. Immediately downstream of the COX1 gene is the ATPase subunit 8 gene (A8) that shows 82.6% base matching to its counterpart in S. cerevisiae mtDNA.