Deficiency in mRNA splicing in a cytochrome c mutant of neurospora crassa: importance of carboxy terminus for import of apocytochrome c into mitochondria.

Molecular cloning and characterization of cytochrome c cDNA clones of Neurospora crassa wild-type (74A) and a cytochrome c-deficient mutant (cyc1-1) are described. Southern blot analysis of genomic DNA indicates that only one cytochrome c gene exists in the N. crassa genome. The cDNA sequence of the wild-type cytochrome c confirmed the previously determined protein sequence. Sequence analysis of the cyc1-1 cDNA for cytochrome c revealed the presence of a larger open reading frame, owing to the presence of an unspliced intron in the 3' end of the coding region. Splicing of this intron is obviously prevented due to the presence of two base exchanges in the highly conserved intron consensus sequences. Consequently, cyc1-1 synthesizes apocytochrome c with an altered carboxy terminus, 19 amino acids longer than the wild-type cytochrome c, with the final 27 amino acids being of an unrelated sequence. This alteration in the carboxy terminus renders the apocytochrome c incompetent for binding to mitochondria and, consequently, import into mitochondria. Thus, unlike other mitochondrial precursor proteins, where it has been demonstrated that the amino terminus alone is sufficient to target the protein to the mitochondria, an intact carboxy terminus is required for efficient import of apocytochrome c into mitochondria. This is independent confirmation for the view that the import pathway of cytochrome c is unique with respect to all other mitochondrial proteins studied to date.