A temperature-sensitive mutant of Neurospora crassa deficient in cytochrome b.

Summary A nuclear gene mutant of Neurospora crassa designated cyb-3 is deficient in cytochrome b and coenzyme QH₂-cytochrome c reductase. Nearly normal when grown at 25°C, the strain expresses a mutant phenotype at 38°C. Mitochondria from cybr-3 mycelium, which has undergone 3–4 mass doublings at the elevated temperature, possess 3-fold less cytochrome b, 2-fold more cytochrome, c, 5-fold less coenzyme QH₂-cytochrome c reductase activity, and require 3-fold less antimycin A per milligram of protein to inhibit NADH oxidation than do wild type mitochondria. The activity of coenzyme QH₂-cytochrome c reductase declines rather slowly in cultures of cyb-3 transferred to 38° C, and the in vitro thermostability of the enzyme is very similar in wild type and mutant mitochondria. Therefore, the mutation may decrease synthesis or impair integration into the membrane of cytochrome b and perhaps other proteins of the enzyme complex.Contribution No. 1294-j, Division of Biology, Agricultural Experiment Station, Kansas State University, Manhattan, Kansas.