We have determined the DNA sequence of the wild type and mutated introns as well as their flanking exons in the yeast mitochondrial gene specifying cytochrome b. The second intron (box3) encodes a trans-acting protein XmRNA maturaseX responsible for splicing and maturation of cytochrome b mRNA. This protein is interlaced with cytochrome b exon sequences. Its biosynthesis is subject to a negative feedback which may constitute a regulatory mechanism for the expression of split genes.