Cytochrome b in Neurospora crassa Mitochondria

Cytochrome b was purified from Neurospora crassa mitochondria by means of chromatography on oleyl‐polymethacrylic acid resin, chromatography on DEAE‐cellulose and recycling gel filtration on Sephadex G‐75, all steps being performed in a medium containing bile acids and salts. The heme content of the preparation was calculated on the basis of incorporated 59 Fe and [ 3 H]‐leucine to be 33–40 μmol/g protein, corresponding to a minimum molecular weight of 25000 to 30000. Upon gel filtration on Sephadex G‐75 in a bile acid and KCl medium the preparation migrated like a 55000‐molecular‐weight protein. Upon gel electrophoresis on polyacrylamide or upon gel filtration on Sephadex G‐100 in a dodecylsulfate medium the protein part of the preparation migrated as one band of the apparent molecular weight of approximately 30000. These findings suggest that the purified cytochrome b is a dimeric heme protein. In vivo incorporation of [ 3 H]leucine into the protein part of the purified cytochrome b was insensitive to cycloheximide but sensitive to chloramphenicol. This indicates that the apoprotein of cytochrome b is translated on mitochondrial ribosomes.