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

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