<i>S. cerevisiae</i> Mrs1 is incompatible with an ancient <i>COX1</i> intron existing in the <i>S. paradoxus</i> and <i>S. bayanus</i> mitochondrial genomes.

<p>(A) <i>S. paradoxus</i> or <i>S. bayanus mrs1Δ</i> mutants carrying <i>Sc-MRS1</i> are unable to produce mature <i>COX1</i> mRNA. Total RNA isolated from the <i>mrs1Δ</i> mutants carrying various orthologous <i>MRS1</i> genes was used for Northern blotting. <i>MRS1</i> function is involved in splicing the introns of <i>COX1</i> and <i>COB</i>, but the incompatibility caused by <i>Sc-MRS1</i> does not affect <i>COB</i> mRNA maturation. <i>COX2</i> mRNA was used as a loading control. (B) <i>COX1</i> mRNA is not translated in the <i>S. paradoxus</i> or <i>S. bayanus mrs1Δ</i> mutants carrying <i>Sc-MRS1</i>. Mitochondria were purified from the <i>mrs1Δ</i> mutants carrying various orthologous <i>MRS1</i> genes. The mitochondrial translation products were labeled with [<sup>35</sup>S]-methionine in the presence of cycloheximide and resolved by electrophoresis on a 17.5% SDS-polyacrylamide gel. (C) Comparison of the <i>COX1</i> introns between different yeast species. Species are arranged in order according to their phylogenetic distance from <i>S. cerevisiae</i>. Solid triangles represent introns and the numbers mark their positions in the <i>COX1</i> mRNA. M1 and M2 are the introns that require Mrs1 for their excision.</p>