Cytochrome c1 is a component of the mitochondrial respiratory chain in most eukaryotes. The protein is coded by nuclear DNA, synthesized as a larger precursor outside the mitochondria and then cleaved to the mature form in two successive steps during its import into the mitochondria. We have cloned the structural gene for yeast cytochrome c1 by functional complementation of a cytochrome c1‐deficient yeast mutant with a yeast genomic library in the yeast‐Escherichia coli ‘shuttle’ vector YEp 13. The complete nucleotide sequence of the gene and of its 5′‐ and 3′‐flanking regions was determined. The deduced amino acid sequence of the yeast cytochrome c1 precursor reveals an unusually long transient amino‐terminal presequence of 61 amino acids. This presequence consists of a strongly basic amino‐terminal region of 35 amino acids, a central region of 19 uncharged amino acids and an acidic carboxy‐terminal region of seven amino acids. This tripartite structure of the presequence resembles that of the precursor of cytochrome c peroxidase and supports a previous suggestion on the import pathways of these two precursors.
Mitochondrial mutants of Succhuromyces cerevisiue defective in cytochrome b were analyzed genetically and biochemically in order to elucidate the role of the mitochondrial genetic system in the biosynthesis of this cytochrome.The mutants mapped between OLIl and OLZ2 on mitochondrial DNA in a region called COB. A fine structure map of the COB region was constructed by rho-deletion mapping and recombination analysis.The combined genetic and biochemical data indicate that the COB region is mosaic and contains at least five distinct clusters of mutants, A-E, with A being closest to OLZ2 and E being closest to OLIl. Clusters A, C and E are probably coding regions for apocytochrome b, whereas clusters B and D seem to be involved in as yet unknown functions. These conclusions rest on the following evidence.1. Most mutants in clusters A, C and E have specifically lost cytochrome b. Many of them accumulate smaller mitochondrial translation products ; some of these were identified as fragments of apocytochrome b by proteolytic fingerprinting. The molecular weight of these fragments depends on the map position of the mutant, increasing in the direction OLI2+OLII. The mutant closest to OLIl accumulates an apocytochrome b which is slightly larger than that of wild type.2. A mutant in cluster C exhibits a spectral absorption band of cytochrome b that is shifted 1.5 nm to the red.3. Mutants in clusters B and D are pleiotropic. A majority of them are conditional and lack the absorption bands of both cytochrome b and cytochrome uu3; these mutants also fail to accumulate apocytochrome b and subunit I of cytochrome c oxidase and instead form a large number of abnormal translation products whose nature is unknown.4. Zygotic complementation tests reveal at least two complementation groups: The first group includes all mutants in cluster B and the second group includes mutants in clusters (A + C + D + E).
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