Conversion of a Mitochondrial Precursor Polypeptide into Subunit 1 of Cytochrome Oxidase in the mi‐3 Mutant of Neurospora crassa

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Abstract
The cytochrome-aa3-deficient mi-3 cytoplasmic mutant of N. crassa synthesizes a mitochondrial translation product which cross-reacts with antibodies specific to subunit 1 of cytochrome oxidase. The immunoprecipitated polypeptide migrates more slowly during gel electrophoresis than the authentic 41,000-MW subunit 1 of the wild-type enzyme. An apparent MW of about 45,000 was estimated for the mutant product. Radioactive labeling experiments in vivo show that the cross-reacting material found in the mutant is relatively stable and does not form complexes with other subunits of the oxidase. After induction of a functional cytochrome oxidase in the mutant cells with antimycin A, the 45,000-MW polypeptide is converted to a 41,000-MW component, which exhibits the same electrophoretic mobility as subunit 1 of the oxidase. Pulse-chase labeling kinetics reveal a typical precursor product relationship. The converted polypeptide becomes assembled with other enzyme subunits to form a protein complex which has the immunological characteristics of cytochrome oxidase. A possible physiological role of the post-translational processing of the mitochondrially synthesized component is discussed.