Mitochondrial DNA endonuclease and mRNA maturase involved in intron homing and required for splicing of the cytochrome b (cobA) gene intron, containing its own coding sequence. The protein stimulates the intrinsic ribozyme activity of the intron through binding to and stabilizing specific secondary and tertiary structure elements in the RNA. As an endonuclease it introduces a specific double-strand break at the junction of the two exons the cobA 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'-GAGGAGGTTTCTCTGTA-3'. The proteins RNA and DNA recognition and binding surfaces are independent. Homodimer. The mature protein may arise from proteolytic cleavage of an in-frame translation of cobA exon 1 plus intron, containing the I-AniI open reading frame. Cleavage may take place close to Met-213 resulting in an active endonuclease/maturase of about 30 kDa (By similarity). Residues 237 to 488 are sufficient for endonuclease, intron homing, and RNA maturase activity. In the C-terminal section; belongs to the LAGLIDADG endonuclease family. The cobA ORF was not predicted to be expressed alternatively as a fusion with intron 4, and intron 4 CDS was not annotated.