RNA IN PLANT VIRUSES 25 



cell constituents grow at the same rate, the protein turnover is 

 identical in all types of granules. Therefore, in the case of micro- 

 organisms kept continuously in the exponential phase of growth, 

 microsomes cannot be the only source of cytoplasmic proteins. 



Comparable observations have been made, with liver, by Khesin 

 and Petrashkaite (1955). They claim that, as in Webster's (1956) 

 case, isolated cytoplasmic granules are capable of net protein syn- 

 thesis. But, while in adult animals mitochondria show the same 

 activity as microsomes, the former are said to be more active than 

 the latter when homogenates from livers of young organisms are 

 considered. Here again, it seems that microsomes are not the only 

 source of cytoplasmic proteins. 



Such a conclusion is in agreement v/ith the fact reported earlier 

 that in leaves microsomes are no more active than chloroplasts in 

 incorporating amino acids into proteins (Stephenson and Zamecnik, 

 1956). Similar observations have also been made with muscle where, 

 according to McLean et al, (1956), microsomes and mitochondria 

 are of equal importance for protein synthesis. 



In fact, more recent work by McLean et al. (1958) shows that 

 incorporation of amino acids into mitochondria is stimulated rather 

 than inhibited by ribonuclease. They also claim that these mitochon- 

 dria are even capable of a net synthesis of cytochrome C. 



There is thus no doubt that the microsomes, especially the small 

 RNA-rich granules which they contain, play an essential part in 

 protein synthesis. However, it would be a mistake to believe that 

 they are the only possible site of cytoplasmic protein synthesis. 



5. THE ROLE OF RNA IN PLANT VIRUSES 



All the plant viruses which have been purified and in many instances 

 crystallized, contain large amounts of RNA, ranging from 6% 

 (tobacco mosaic virus) up to 35% (turnip yellow mosaic virus). 

 RNA in plant viruses stands in close association with a simple pro- 

 tein, which is devoid of enzymatic activity. 



The fact that such simple nucleoproteins are able to reproduce 

 themselves and are thus endowed with genetic continuity is of the 



References p. 50/54 



