Sequence of introns and flanking exons in wild-type and box3 mutants of cytochrome b reveals an interlaced splicing protein coded by an intron.

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 "mRNA maturase" 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.