Two homologous introns from related Saccharomyces species differ in their mobility

We have studied gene conversion initiated by the ai3 intron of the Saccharomyces cerevisiae mitochondrial (mt) COXI gene and its homologous intron (S.cap.ai1) from Saccharomyces capensis. The approach used involved the measurement of intron transmission amongst the progeny of crosses between constructed recipient and donor strains. We found that the S. cerevisiae ai3 intron is extremely active as a donor in gene conversion, whereas its homologous S. capensis intron is not. We have established the sequence of S.cap.ai1 and compared its open reading frame (ORF) with that of I-SceIII encoded by the homologous S. cerevisiae intron. The two protein-coding intron sequences are almost identical, except that the S. capensis ORF contains an in-frame stop codon. This finding provides a strong indication that the 3' part of the S. cerevisiae intron ORF encoding I-SceIII (which should not be translated in the S. capensis intron) must be critical for function of mtDNA endonucleases to mediate intron mobility.