Mitochondrial mRNA maturase required for splicing of intron 3 of the cytochrome b (COB) gene, containing its own coding sequence. Encoded from partially processed COB mRNA that terminates with the in-frame coding sequence of the third intron. In the N-terminal section; belongs to the cytochrome b family. In the C-terminal section; belongs to the LAGLIDADG endonuclease family.