Mitochondrial mRNA maturase required for splicing of intron 4 of the cytochrome b (COB) gene, containing its own coding sequence, and intron 4 in COX1, coding for the related homing endonuclease aI4. In vivo splicing requires in addition the imported mitochondrial leucyl-tRNA synthetase NAM2. Both proteins seem to stimulate the intrinsic ribozyme activity of intron bI4 through binding to and stabilizing specific secondary and tertiary structure elements in the RNA. Forms a ternary complex with intron derived RNA and the imported mitochondrial leucyl-tRNA synthetase NAM2. The proteins do not interact directly with each other. The mature protein may arise from proteolytic cleavage of an in-frame translation of COB exons 1 to 4 plus intron 4, containing the bI4 open reading frame. Cleavage would take place close to the Met-385 resulting in an active maturase of about 30 kDa. Residues 385 to 638 are sufficient for maturase activity. In the C-terminal section; belongs to the LAGLIDADG endonuclease family. COB was not predicted to be expressed alternatively as a fusion with intron 4. COB was not predicted to be expressed alternatively as a fusion with intron 4.