37. nucleic acid and protein synthesis 395 



4. The Mitochondria and Chloroplasts 



A number of investigations have made it clear that the ribosome in 

 its usual anatomical situation is not the only cellular site of protein syn- 

 thesis. In vivo and in vitro studies have shown that nuclei, cell membrane 

 material in bacteria, plant chloroplasts, and plant and animal mitochon- 

 dria are all capable of incorporating amino acids into their proteins. We 

 shall briefly consider these possible other sites of protein synthesis insofar 

 as they shed light on RNA-protein synthesis relations. 



In none of these experiments has the evidence for autonomous protein 

 synthesis been so convincingly marshaled as in the studies of Simpson and 

 co-workers. 29 • 202 These workers have shown that calf heart mitochondria 

 are capable of net synthesis of an intrinsic protein-cytochrome c — when 

 an energy source and an amino acid mixture are supplied. They have es- 

 tablished that there is a net increase in new enzyme protein gravimetrically, 

 by isotope dilution technique and by isolating peptide fragments of the 

 protein into which a C 14 -amino acid has become incorporated. (It is of in- 

 terest in respect to this latter study that valine-C 14 differs in specific ac- 

 tivities in two peptides derived from different parts of cytochrome c.) 

 Pertinent to our subject is the finding that an extract of sonically disrupted 

 mitochondria is still capable of incorporating labeled amino acids into 

 cytochrome c, and this reaction is sensitive to ribonuclease. 203 The extract 

 contains RNA and it will be of great interest to follow developments in 

 this story to see whether reaction pathways similar to those found in the 

 transfer RNA-ribosome system are found. 



Greengard and Campbell 204 have studied the effect of added soluble 

 components on incorporation reactions in mitochondria. They have 

 found, in agreement with an earlier report of McLean et aL, 205 that ribo- 

 nuclease had no effect on amino acid incorporation in intact mitochondria. 

 The addition of "pH 5 fraction" (containing transfer RNA) to mitochon- 

 dria does produce some stimulation of incorporation which is ribonuclease 

 sensitive. This suggests the possibility that transfer RNA might be diffus- 

 ible into the mitochondria and participate in the synthetic reaction. 



Although it is known that chloroplasts contain RNA 12 and that they in- 

 corporate amino acids into protein, 206 little is yet known about the chemi- 

 cal events involved. 



201 J. T. Park and J. L. Strominger, Science 125, 99 (1957). 



202 H. M. Bates and M. V. Simpson, Biochim. et Biophys. Acta 32, 597 (1959). 



203 M. V. Simpson, unpublished data, (1959). 



204 O. Greengard and P. N. Campbell, Biochem. J. 72, 305 (1959). 



205 J. R. McLean, G. L. Cohn, I. K. Brandt, and M. V. Simpson, J. Biol. Chem. 233, 

 657 (1958). 



206 M. L. Stephenson, K. V. Thimann, and P. C. Zamecnik, Arch. Biochem. Biophys. 

 65, 194 (1956). 



