A nuclear mutant of Neurospora crassa lacking subunit 1 of cytochrome c oxidase.

A new, nuclear, cytochrome-deficient mutant of Neurospora crassa has been isolated. The mutant is characterized by its slow growth rate, high dependence on the alternate oxidase pathway of respiration, absence of spectrally detectable cytochrome aa3, and low levels of cytochrome c oxidase activity. The amount of cytochrome b appears to be normal in the mutant whereas the amount of cytochrome c is elevated approximately 2-fold, compared to the amounts found in wild type. Sodium dodecyl sulfate-gel electrophoretic analysis of immunoprecipitates obtained from the mitochondria of the mutant using antibody directed against holocytochrome c oxidase revealed that only subunits 5 and 6 of the enzyme could be detected by this method. Mitochondria isolated from cells labeled in the presence of cycloheximide were examined using antibodies specific to each of the mitochondrially synthesized subunits of the enzyme. This analysis revealed a complete absence of immunoprecipitable subunit 1 in the mitochondria of the mutant. Subunits 2 and 3 were present, although the amount of subunit 2 was somewhat lower than the amount found in mitochondria isolated from wild type grown under similar conditions. The amount of subunit 3 in the mutant was comparable to the amount found in wild type. Since there is no direct evidence which shows that the gene for subunit 1 is found on mtDNA, the possibility that the nuclear mutation directly affects the polypeptide sequence of the subunit cannot be completely eliminated. However, if it is assumed that mitochondrial translation products are encoded by mtDNA, then the fact that a nuclear mutation gives rise to a deficiency of a mitochondrially synthesized polypeptide suggests that a regulatory entity necessary for the production of subunit 1 of cytochrome c oxidase may be missing or altered in the mutant described here.