Mitochondrial DNA endonuclease involved in intron homing. The mature protein may arise from proteolytic cleavage of an in-frame translation of COX1 exons 1 to 5 plus intron 5, containing the aI5 open reading frame. In the C-terminal section; belongs to the LAGLIDADG endonuclease family. In the N-terminal section; belongs to the heme-copper respiratory oxidase family.