Mitochondrial mRNA maturase required for splicing of intron 3 of the cytochrome b (COB) gene, containing its own coding sequence. In vivo splicing requires the formation of a ribonucleoprotein complex together with the imported mitochondrial RNA-splicing protein MRS1. The complex seems to stimulate the intrinsic ribozyme activity of intron bI3 through binding to and stabilizing specific secondary and tertiary structure elements in the RNA. Forms a ribonucleoprotein complex composed of maturase bI3 and 2 dimers of MRS1 that assemble around the bI3 RNA. 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.