Mitochondrial DNA endonuclease involved in intron homing. Endonuclease aI5 beta (group I intron) is encoded within an intron of the mitochondrial COX1 gene. Splicing from COX1 requires PET54, MRS1, SUV3 and MSS18. Translational initiation codon is predicted to be ATA rather than ATG. Belongs to the LAGLIDADG endonuclease family.