Critical sequences within mitochondrial introns: pleiotropic mRNA maturase and cis-dominant signals of the box intron controlling reductase and oxidase.

We have established the DNA sequence of nine yeast mutants that prevent the expression either of the split cytochrome b gene alone (five mutants) or of two split genes, the cytochrome b gene and the cytochrome oxidase subunit I gene (four mutants). All the mutations analyzed are localized in intron 14 of the cob-box gene. We have extended the concept of the intron-encoded mRNA maturase, already described for intron 12, to the intron 14, and have adduced evidence that this box7 pleiotropic maturase is involved in the splicing of two distant gene transcripts. Such a process may constitute a regulatory mechanism that coordinates the expression of two structurally nonhomologous genes encoding two metabolically related enzymes. Analyses of cis-dominant mutations reveal the role of signal sequences in the recognition of the intron RNA sequences to be excised. These signal sequences are localized near the exon-intron boundaries (box1), or quite distant from the splicing sites, either in the blocked reading frame (box2) or in the open reading frame (box9) of the intron. We believe that for the last sequence, a ribosomal recognition of the box9 signal could be involved in a regulatory mechanism of the splicing of the pre-mRNA.