SUBUNIT-VII OF UBIQUINOL-CYTOCHROME-C OXIDOREDUCTASE FROM NEUROSPORA-CRASSA IS FUNCTIONAL IN YEAST AND HAS AN N-TERMINAL EXTENSION THAT IS NOT ESSENTIAL FOR MITOCHONDRIAL TARGETING

cDNA clones encoding subunit VII of the Neurospora crassa bc(1) comple x (ubiquinol: cytochrome-c oxidoreductase), which is homologous with s ubunit VIII of the complex from yeast (encoded by QCR8), were identifi ed on the basis of functional complementation of a yeast QCR8 deletion strain. The clones contain an open reading frame encoding a protein w ith a calculated molecular mass of 11.8 kDa. The N-terminal eight resi dues of the amino acid sequence deduced from the cDNA clones are absen t from the mature protein, as revealed by direct sequencing of the iso lated protein. To investigate the potential role of the N-terminal oct apeptide in mitochondrial targeting, constructs were made encoding the precursor and the mature form of subunit VII from Neurospora. Incubat ion of isolated mitochondria with the two proteins revealed that the N -terminal extension of the precursor is removed on import, However, th e presequence does not encode information for targeting, as the protei ns encoded by both constructs can be imported into isolated mitochondr ia with equal efficiency. In contrast, the octapeptide seems to have f unctional importance: the defect in the yeast qcr8-null mutant is not complemented on transformation with the construct encoding mature subu nit VII from N. crassa in a single-copy plasmid. We therefore speculat e that the N-terminal extension plays a role in intramitochondrial sor ting of N. crassa subunit VII. This is supported by the fact that the subunit VII precursor is processed by a protease other than the genera l mitochondrial processing peptidase. Interestingly, the presequence o f N. crassa subunit VII has an amino acid composition similar to the o ctapeptides cleaved off by the mitochondrial intermediate peptidase.