Mitochondrial DNA endonuclease involved in intron homing. It introduces a specific double-strand break in the DNA of the COX1 gene and thus mediates the insertion of an intron, containing its own coding sequence (group I intron), into an intronless gene. Recognizes with limited specificity and cleaves the sequence 5'-ATCTTCTCTTGATTAGCCCTGATCTACGG-3' after the nucleotide sequence ATTA leaving a four-base overhang and a 3'-OH. Optimum pH is 9.5. Heterodimer of a 32 kDa catalytic subunit and a larger 60 kDa polypeptide subunit. The mature protein may arise from proteolytic cleavage of an in-frame translation of some COX1 exons plus the intron containing the aI5_alpha open reading frame. In the N-terminal section; belongs to the heme-copper respiratory oxidase family. In the C-terminal section; belongs to the LAGLIDADG endonuclease family.