54 THE BIOSYNTHESIS OF PROTEINS 



locating, among the mitochondrial components, the first centres of protein 

 synthesis. The results at the time of writing are not quite clearcut. Rendi 

 (1959, 1960) has isolated from disrupted mitochondria a sedimentable 

 fraction which contains most of the RNA of the mitochondria and may 

 have some similarity with ribosomes. On the other hand, Kalf and Simpson 

 (1959) showed that when mitochondria are exposed to sonic vibrations 

 followed by differential centrifugation, components which are not sedi- 

 mented in 8 hrs at 105,000g possess a pronounced capacity to incorporate 

 amino acids into protein material. The protein synthesizing system in this 

 supernatant must comprise RNA as an essential part, for ribonuclease 

 suppresses the amino acid incorporation (see also Kalf et ah, 1959). The 

 discrepancy between these two results probably reflects differences in 

 degree of disorganization of the preparations used. Both groups of work, 

 however, point to some RNA containing element as being again at the 

 heart of the process of protein synthesis. 



3. Protein Synthesis in Chloroplasts 



In green plant tissues, much less information has been obtained so far 

 on the kinetics of amino acid incorporation. Stephenson et al. (1956) and 

 Sissakian (1957) studied the incorporation of labelled amino acids in vivo 

 into various cell constituents of tobacco leaves and found that microsomes 

 are preferentially labelled and contain the bulk of radioactivity for the first 

 few minutes, but that chloroplasts thereafter incorporate amino acids at a 

 fairly high rate, as if they were also important centres of protein synthesis. 

 Plashkov and Ivanko (1956) found the highest in vivo incorporation of 

 methionine into chloroplasts and mitochondria. Racusen and Hobson 

 (1959) using ^^002 as the precursor found no significant difference between 

 the synthesis of chloroplast protein and the other proteins, in swiss chard 

 leaves. 



That chloroplasts and microsomes contain independent centres of 

 protein synthesis was actually established by earlier data which passed 

 unnoticed (Chantrenne et al., 1953 ; Brachet et al., 1955). In parallel experi- 

 ments in which 1^002 and ^^lycine were used as precursors, it was found 

 that glycine incorporation is more rapid in microsomes than in chloroplasts, 

 whereas i'*C02 to the contrary is more rapidly incorporated into the 

 proteins of the chloroplasts than into microsomal protein. This clearly 

 indicates that amino acids which arise within the chloroplasts as a result 

 of photosynthetic carbon dioxide fixation can be used for protein synthesis 

 directly within the chloroplast without going through the microsomes. 

 Since, on the other hand, an exogenous amino acid is incorporated more 

 readily in the microsomes than into the chloroplasts, it must be concluded 

 that microsomes and chloroplasts are two independent sites of protein 

 synthesis. 



