Transposition of an intron in yeast mitochondria requires a protein encoded by that intron

The optional 1143 by intron in the yeast mitochondrial 21S rRNA gene (ω⁺) is nearly quantitatively inserted in genetic crosses into 21S rRNA alleles that lack it (ω^-). The intron contains an open reading frame that can encode a protein of 235 amino acids, but no function has been ascribed to this sequence. We previously found an in vivo double-strand break in ω^- DNA at or close to the intron insertion site only in zygotes of ω⁺ × ω^- crosses that appears with the same kinetics as intron insertion. We now show that mutations in the intron open reading frame that would alter the translation product simultaneously inhibit nonreciprocal ω recombination and the in vivo double-strand break in ω^- DNA. These results provide evidence that the open reading frame encodes a protein required for intron transposition and support the role of the double-strand break in the process.