Aprotein complex required for the splicing of intron 3 of the cytochrome b (COB) gene. In S. cerevisiae, the complex contains the maturase bI3 (which derives from one of the products of the splicing), the MRS1 cofactor and the intron 4 of the cytochrome b pre-mRNA. The two proteins stimulate the ribozyme activity of the pre-mRNA which autoctalyse a group I intron splicing.
bI3 intron splicing complex