Mitochondrial DNA endonuclease involved in intron homing. It introduces a specific double-strand break at the junction of the two exons a4-a5 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'-TTTGGTCACCCTGAAGTA-3'. The protein may acquire mRNA maturase activity, like the closely related bI4, through a single amino acid substitution Glu-362 to Lys or when present together with a mutant form of the imported mitochondrial leucyl-tRNA synthetase NAM2. Homodimer. The mature protein may arise from proteolytic cleavage of an in-frame translation of COX1 exons 1 to 4 plus intron 4, containing the aI4 open reading frame. Cleavage would take place close to the Met-299 resulting in an active endonuclease of about 30 kDa. Residues 299 to 556 are sufficient for endonuclease and intron homing activity. In the C-terminal section; belongs to the LAGLIDADG endonuclease family. In the N-terminal section; belongs to the heme-copper respiratory oxidase family. COX1 was not predicted to be expressed alternatively as a fusion with intron 4. COX1 was not predicted to be expressed alternatively as a fusion with intron 4.