Deletion of the COX7 gene in Saccharomyces cerevisiae reveals a role for cytochrome c oxidase subunit VII In assembly of remaining subunits

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Abstract
Summary: Cytochrome c oxidase from Saccharomyces cerevisiae is composed of nine subunits. Subunits I, II and III are products of mitochondrial genes, while sub‐units IV, V, VI, VII, Vila and VIII are products of nuclear genes. To investigate the role of cytochrome c oxidase subunit VII in biogenesis or functioning of the active enzyme complex, a null mutation in the COX7 gene, which encodes subunit VII, was generated, and the resulting cox7 mutant strain was characterized. The strain lacked cytochrome c oxidase activity and haem a/a3 spectra. The strain also lacked subunit VII, which should not be synthesized owing to the nature of the cox7 mutation generated in this strain. The amounts of remaining cytochrome c oxidase sub‐units in the cox7 mutant were examined. Accumulation of subunit I, which is the product of the mito‐chondrial COX1 gene, was found to be decreased relative to other mitochondrial translation products. Results of pulse‐chase analysis of mitochondrial translation products are consistent with either a decreased rate of translation of COX1 mRNA or a very rapid rate of degradation of nascent subunit I. The synthesis, stability or mitochondrial localization of the remaining nuclear‐encoded cytochrome c oxidase subunits were not substantially affected by the absence of subunit VII. To investigate whether assembly of any of the remaining cytochrome c oxidase subunits is impaired in the mutant strain, the association of the mitochondrial‐encoded subunits I, II and HI with the nuclear‐encoded subunit IV was investigated. Relative to the parental COX7+ strain, the cox7 mutant strain showed no association of sub‐unit IV with subunit I, and decreased association of subunits IV and II. These results provide direct evidence for a role for subunit VII in assembly between mitochondrial‐encoded and nuclear‐encoded cytochrome c oxidase subunits.

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