The morphological site of synthesis of cytochrome c in mammalian cells (Krebs cells).
In Krebs ascites-tumour cells, cytochrome c is segregated in the mitochondria and the level in microsomes could not be measured. At 22 degrees in glucose-buffer Krebs cells synthesized a spectrum of proteins including cytochrome c. Mild osmotic shock in the presence of ribonuclease had little effect on incorporation of [(14)C]-leucine or [(14)C]valine into mixed mitochondrial protein but strongly inhibited synthesis of non-mitochondrial cytoplasmic proteins. Under these conditions, labelling of cytochrome c was also strongly inhibited. After pulse labelling of Krebs cells at 22 degrees for 10min. the cytcchrome radioactivity found in mitochondria was higher than in microsomes. After addition of unlabelled amino acid as ;chase' there was 137% increase in radioactivity of cytochrome c but only a 3% increase in radioactivity of whole-cell protein. It is concluded that the peptide chain of cytochome c is synthesized on cytoplasmic ribosomes. Mitochondria therefore do not have the character of self-replicating entities, but are formed by the cooperative function of messenger RNA of cytoplasmic ribosomes and, possibly, of intramitochondrial messenger derived from the mitochondrial DNA.
Freeman
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KB
National Institute for Medical Research, Mill Hill, London, N.W. 7.
Haldar
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Work
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