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 high specificity and cleaves the sequence 5'-GGTTTTGGTAACTATTTATTAC-3'. Optimum pH is 8.0. The mature protein may arise from proteolytic cleavage of an in-frame translation of some COX1 exons plus the intron containing the aI3 open reading frame. Strain Capensis / YB4237 has two stop codons in position 276 and 407, which disrupt the gene coding for this protein. Consequently, the corresponding intron containing its coding sequence is not mobile. In the C-terminal section; belongs to the LAGLIDADG endonuclease family. Truncated C-terminus.