22 



RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



and Morton (1956), who worked on plant material. It thus seems to 

 be a general property of all living organisms that microsomes, 

 probably because of their high RNA content, play an exceedingly 

 important role in protein synthesis. 



Experiments by Allfrey et al. (1953) and by Zamecnik and Keller 

 (1954) have given proof that RNA is directly involved in the in- 

 corporation of amino acids into the proteins of the microsomes : 

 adding ribonuclease strongly inhibits the in vitro incorporation 

 process. According to Zamecnik and Keller (1954), all that is re- 

 quired for the incorporation are microsomes with intact RNA, a 

 non-dialyzable soluble fraction, and an ATP-generating system. 

 We shall return to this point later in this chapter. We have seen 

 that the microsomes can be disintegrated by deoxycholate, with the 



40 



Deoxycholate 

 soluble 



Deoxycholate 

 insoluble 



Soluble protein 

 of cell 



15 20 2j 



Time in min 



Fig. 9. Incorporation in vivo of leucine into the two components of the micro- 

 somes and into the soluble protein of the cell (Littlefield et al., 1955). 



liberation of small ribonucleoprotein particles (about 240 A diam- 

 eter) as a result. According to Littlefield et al. (1955) and Zamecnik 

 et al. (1956), these small granules, which contain as much as 44% 

 RNA and can be considered as simple nucleoproteins, are seven to 

 eight times more active as regards amino acid incorporation than 

 the deoxycholate-soluble material from the microsomes (Fig. 9). 



