Mitochondrial protein-synthesizing activity

The ribosomes of chloroplasts and mitochondria are similar in size and function to those of prokaryotes. For example, drugs such as the antibiotic chloramphenicol, which kill certain bacteria by inhibiting the protein-synthesizing activities of ribosomes, also inhibit chloroplast and mitochondrial ribosomal function. 

If radioactive amino acids are incorporated into mitochondrial proteins in vitro radioactivity is associated with mitoribosomes (Ashwell and Work, 1970). But this activity can be removed by puromycin treatment, suggesting that the radioactivity is the result of incomplete peptide chains on the mitoribosomes. There is no evidence that any mitochondrial ribosomal proteins are labeled in vitro although a clear-cut separation of the ribosomal proteins from animal cells has not been reported. Even with N. crassa no evidence was found that ribosomal proteins are coded for by mt DNA (Lizardi and Luck, 1972). From all these data it seems very likely that mitochondrial proteins are synthesized outside the mitochondria, under control of the nuclear genome, and are subsequently transported into the mitochondrial compartment. 

The kinetic and equilibrium parameters of L-malate, succinate, citrate, and a-oxoglutarate uptake have been determined in mitochondria isolated from respiratory-competent cells grown under conditions of aerobic derepression, aerobic and anaerobic catabolite repression, and inhibition of mitochondrial protein synthesis, and also in mitochondria prepared from a respiratory-deficient cytoplasmic petite strain. The activity and kinetic characteristics of the systems were similar in all cases. It may be concluded that the protein components of these transport systems are coded entirely by nuclear DNA and are synthesized on the cytoplasmic ribosomes. 

Although mitochondria possess a protein-synthesizing system active both in vivo and in vitro, it is clear that the vast majority of mitochondrial proteins are synthesized on cytoplasmic ribosomes and are coded for by nuclear DNA. This conclusion is based, in part, on the observations that 85-95% of mitochondrial protein can be labeled with radioactive amino acids in the presence of chloramphenicol, but not cyclohexmide, and that only small qualitative differences can be detected in mitochondrial proteins between respiratory-competent yeast cells and cytoplasmic petites. ... 

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