404 N. M. SISAKYAN 



anaerobic œnditions, maximal incorporation of label is found when the micro- 

 somal fraction is combined with the supernatant. 



In all these experiments the incorporation of labelled amino acids in the 

 proteins of the isolated structures served as a criterion of the formation of peptide 

 bonds. 



In the work of other authors [17] attempts have been made to study the syn- 

 thesis of protein by another method — by determining the increase of activit}' of 

 a particular enzyme (the amylase of the pancreas). According to Khesin's data, 

 the synthesis of protein is brought about in large, light granules by the agency 

 of factors produced by the mitochondria. 



Recent investigations [18, 19] have shown that, under special circumstances, 

 radioactive labels are incorporated in vitro into the protein of the cellular 

 nucleus, the increase in radioactivity being associated, not with exchange of the 

 amino acids of the protein for amino acids of the surrounding medium, but 

 with synthesis of protein de novo. 



As concerns the structure of plant cells, it has been shown in Webster's 

 laboratory [20, 21] that the synthesis of peptide bonds of peptides of low mole- 

 cular weight takes place in the mitochondria of plant cells and so does the 

 incorporation of labelled amino acids into the protein of these structures. 



Webster [21] later demonstrated the incorporation of labelled amino acids 

 into the protein of other structural elements in the cells of etiolated plants. 

 Webster's experiments provide evidence for the participation of the mito- 

 chondria, microsomes and nuclei in the protein synthesis of the cell, incorpora- 

 tion occurring most effectively in particles precipitated from the homogenate of 

 etiolated plants during centrifugation in the region of 10,000-40,000 g. The 

 following fraction, composed of smaller particles, brings about the incorporation 

 of labels at a considerably slower rate. Just as in the experiments of Siekevitz 

 [15] and Keller [14] the incorporation is most effective when the mitochondria 

 and microsomes are incubated together. It would seem that these findings allow 

 us to accept the existence of general features of the locaHzation of protein syn- 

 thesis in animal and plant cells. It must, however, be emphasized that Webster's 

 findings refer only to the structural elements of etiolated plants. This being so 

 we cannot yet be sure that this type of locaHzation of protein synthesis within 

 the cell is the same in all vegetable organisms. 



In this connection the study of protein synthesis in the cells of chlorophyll- 

 containing plants is of special interest. The chloroplasts, which are the essential 

 agents of photosynthesis in the plant cell, attract particular attention. 



The possibiUty of the synthesis of peptide bonds by plastids has been studied 

 in our laboratory [2]. It was found that when the plants are etiolated a marked 

 depletion of the enzymes of the plastids takes place while, when they are exposed 

 to light, a very sharp increase in the enzymic activity of the plastids is observed. 

 The transition from leucoplasts to chloroplasts, which occurs on exposure to 

 light, is thus accompanied by an increase in the enzymic power of these struc- 

 tures [22-27]. Since then indirect evidence has also been obtained in other 

 experiments [28, 29] of the participation of chloroplasts in protein synthesis 

 and of its connection with photosynthesis. 



