MICROSOMES AND RNA PARTICLES 23 



A similar fraction has also been obtained from ascites tumor 

 cells by Littlefield and Keller (1957). Here again, the small granules 

 are several times more active than the microsomes, and the incorpo- 

 ration process is inhibited by ribonuclease. Full activity requires 

 the presence of ATP, guanosine triphosphate and a soluble enzyme 

 fraction (which will be discussed later). The facts that the small 

 RNA-rich granules are more active than the whole microsomes and 

 that ribonuclease has inhibitory effects leave little doubt that RNA 

 is directly concerned in the incorporation mechanism ; the latter is 

 considered by Littlefield et al. (1955) as an irreversible step in pro- 

 tein synthesis. 



The question of the role of the microsomes in protein synthesis 

 of plant cells has been the object of several interesting studies. 

 According to Stephenson and Zamecnik (1956), the microsomes of 

 leaves are no more active than other cell fractions. Only the chloro- 

 plasts can incorporate amino acids into their proteins in vitro, pro- 

 vided that oxygen and light are supplied. But, in more recent ex- 

 periments, Stephenson et al. (1956) come to the conclusion that in 

 tobacco leaves microsomes are, as in animal cells, initial sites of 

 incorporation of amino acids into proteins. 



While the respective roles of the chloroplasts and the micro- 

 somes in protein synthesis remain obscure, there is no doubt that 

 in plant cells which are free of chloroplasts the microsomes play the 

 same role as in animal cells. The experiments of Webster and 

 Johnson (1955) are very impressive in proving this point. Studying 

 incorporation of amino acids in a particulate portion of pea roots, 

 which is homologous to the microsome fraction, Webster (1955) 

 and Webster and Johnson (1955) found the process to be stimulated 

 by the addition of ATP, Mg ions and a mixture of seventeen amino 

 acids. As in animal cells, the incorporation is inhibited by ribonu- 

 clease and the addition of RNA induces a recovery of the activity. 

 Restoration of glutamate incorporation in the ribonuclease-treated 

 particles could be obtained by the addition of RNA extracted from 

 peas. Addition of RNA prepared from several sources produces a 

 definite stimulation of the incorporation process in the intact 

 particles. The stimulating effect of RNA is thus not specific and it 



References p. 50/54 



