Cytochrome c1, a subunit of the mitochondrial ubiquinol–cytochrome-c reductase, is synthesized on cytosolic ribosomes as a precusor protein of 37 kDa. Maturation to the mature 31-kDa form involves two proteolytic processing steps of the amino-terminal presequnce. After removal of the amino-terminal part by the matrix-localized processing peptidase, the carboxy-terminal part of the presequence is cleaved off by an unknown intermembrane space protease. This step depends on covalent linkage of heme to the apoprotein. At least two complementation groups (I and II) can be distinguished among mutants of the yeast Saccharomyces cerevisiae, which are defective in this second proteolytic processing, i.e. they accumulate the intermediate-sized form of cytochrome c1 instead of for cytochrome c1 [Sadler, I., Suda, K., Schatz, G., Kaudewitz, F. & Haid, A., (1984) EMBO J. 3, 2137–2143]. We report on the molecular cloning and characterization of the CYT2 gene representing complementation group I. It maps on chromosome XI and encodes a mitochondrial protein of about 26 kDa. Extensive similarity to Neurospora crassa and S. cerevisiae cytochrome-c–heme lyase, as well as the phenotype of cyt2 mutants, strogly suggest that we have identified the gene for cytochrome-c1–heme lyase.
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Ubiquinol–cytochrome-c reductase (EC 18.104.22.168)