A regulatory system controlling the production of cytochrome aa3 in Neurospora crassa

Summary The mitochondria of the cyt-2-1, cya-3-16, cya-4-23 and 299-1 nuclear mutants and the mi-3] and exn-5] cytoplasmic mutants of Neurospora crassa are deficient in cytochrome aa ₃, while the cyb-1-1 and cyb-2-1 mutants have mitochondrial b-cytochrome dificiencies. However, the mitochondria from cyb-1-1 cyt-2-1, cyb-1-1 mi-3] and cyb-2-1 mi-3] double mutants contain 30% to 50% of the amount of cytochrome aa ₃ that is present in mitochondria from wild-type; i.e. cyb-1-1 and cyb-2-2 act as suppressors of the cytochrome aa ₃ deficiency phenotypes that are associated with the cyt-2-1 and mi-3] mutations.The production of cytochrome aa ₃ can be induced in cyt-2-1 and mi-3] by growing cells in medium containing antimycin A, an inhibitor of electron transport in the cytochrome bc ₁ segment of the mitochondrial electrontransport chain. Moreover, the growth of the mi-3] mutant is strongly stimulated by low concentrations of antimycin A. The induction of cytochrome aa ₃ by antimycin treatments does not occur in exn-5], cya-4-23 and 299-1 cells, but does take place in cya-3-16 cells.Although some of the seven constituent polypeptides of cytochrome aa ₃ are present the mitochondria of mi-3], the holoenzyme complex is not formed in the mutant. In contrast, the mitochondria of cyb-1-1 mi-3] and cyb-2-2 mi-3] double mutants contain a fully assembled cytochrome oxidase complex as well as some unassembled subunit polypeptides.The observations are indicative of the existence of at least two regulatory systems controlling the production of cytochrome aa ₃. One of the circuits appears to control the basal or constitutive production of cytochrome oxidase, the other seems to coordinate the level of cytochrome aa ₃ with some function of the mitochondrial cytochrome bc ₁ complex, possibly electron transport.