This protein is responsible for splicing and maturation of cytochrome b mRNA. Specifically, it may be responsible for the splicing specificity of the second intron. The variant in strain Capensis / YB4237 has additional DNA endonuclease (I-ScaI) activity promoting intron homing, which makes intron 2 of the COB gene highly mobile (PubMed:8670880). Introduction of 2 (Ala-355 and His-382) of its 4 variant residues into other strains is sufficient for acquisition of endonuclease activity of the corresponding mRNA maturases and for induction of intron mobility (PubMed:8670880). Encoded from partially processed COB mRNA that terminates with the in-frame coding sequence of the second intron. In the N-terminal section; belongs to the cytochrome b family. In the C-terminal section; belongs to the LAGLIDADG endonuclease family.