Cytochrome b in Neurospora crassa Mitochondria A Membrane Protein Containing Subunits of Cytoplasmic and Mitochondrial Origin

1 A membrane protein containing about 16μmol cytochrome b/g protein was prepared from Neurospora crassa mitochondria by means of chromatography on oleyl‐polymethacrylic acid resin. 2 At room temperature the oxidized from absorbs with maxima at 562nm, 530nm and 415nm, the reduced form with maxima at 561 nm, 530 nm and 429 nm. At low temperature these bands are shifted to 561 nm, 525 nm and 414 nm, for the oxidized form and to 558 nm, 527 nm and 427 nm for the reduced form. 3 The apparent molecular weight of this membrane protein is 55000 as determined by means of gel chromatography in a bile acid medium. 4 By means of gel electrophoresis and gel chromatography in sodium dodecylsulfate medium this protein was resolved into two smaller subunits with apparent molecular weights of 10000 and 11000 and into a larger subunit with an apparent molecular weight of 32000. The heme b group was found to be associated with the smaller subunits. 5 The two smaller subunits are synthesized at the cytoplasmic ribosomes, the larger subunit at the mitochondrial ribosomes, as revealed by incorporation of radioactive leucine in the presence of cycloheximide or chloramphenicol, respectively. 6 Pulse‐labeling experiments indicated that the subunits are derived from precursor polypeptides with at least two separate pools. The pool size of the precursor polypeptides of cytoplasmic origin is about 1% of the amount of the corresponding subunits in the membrane protein. The pool size of the precursor polypeptide of mitochondrial origin is about 5%.