Regulation of Mitochondrial Membrane Assembly in Neurospora crassa

Cultures of mutant cni-1, a chromosomal mutant of Neurospora crassa, undergo a marked change in respiratory properties as the age of the culture increases. Early log phase cultures have a high level of respiration that is insensitive to inhibition by cyanide or antimycin A. Late log and stationary phase cultures have reduced rates of respiration. A high percentage of this respiration is inhibited by cyanide. Mitochondria from early log phase k-1 have an excess of cytochrome c and little or no detectable cytochrome uu3. Mitochondria from late log and stationary phase cultures have levels of c-, b-, and a-type cytochromes that are not significantly different in concentration from those found in wild type cells. The cytochrome uu3 content and the cytochrome oxidase activity of b-1 mitochondria increase 5to IO-fold as the age of the culture increases. Mitochondria from early log phase cells of cni-1 synthesize only polypeptides of apparent molecular weights 7,000 to 10,000 and do not synthesize any of the mitochondrial components of cytochrome oxidase. Mitochondria from late log and stationary phase cells synthesize the normal complement of mitochondrial translation products including the mitochondrial components of cytochrome oxidase. The assembly of cytochrome oxidase is likely due to the availability of the mitochondrially synthesized components of the enzyme. The regulation of mitochondrial translation in the b-1 mutant is independent of the nutrient content of the growth medium and is due to the accumulation or depletion of some component within the cell.