Function in mitochondrial RNA splicing in the excision of mitochondrial group I introns aI5 beta from COX1 and bI3 from COB transcripts and thus would be involved in obtaining the correct structure of the intron, to allow the RNA catalyzed reactions to occur. Homodimer. Forms a ribonucleoprotein complex composed of maturase bI3 and 2 dimers of MRS1 that assemble around the bI3 RNA. Present with 5240 molecules/cell in log phase SD medium.