26 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



Utmost importance. Plant viruses are ideal material for the study 

 of the synthesis of specific proteins as well as for the solution of 

 fundamental genetic problems. In recent years, important evidence 

 about the respective role of RNA and proteins in plant virus multi- 

 plication has been discovered. This evidence will now be reviewed. 



In their important studies on turnip yellow mosaic virus, Mark- 

 ham and Smith (1949) were able to separate by ultracentrifugation 

 two distinct components from crystalline preparations of the virus. 

 Both contained serologically identical proteins, but only the more 

 rapidly sedimenting component also contained RNA and proved 

 infective. This finding led Markham (1953) to the important con- 

 clusion that "there is some evidence that the nucleic acid is in fact 

 the substance controlling virus multiplication". Markham and 

 Smith's (1949) findings were soon extended. In 1952, Takahashi and 

 Ishii (1952, 1953) reported the isolation from mosaic-diseased 

 tobacco leaves of an abnormal protein which could be obtained by 

 electrophoresis and which behaved in many ways like tobacco mo- 

 saic virus. Simultaneously, Jeener and Lemoine (1953) and Jeener 

 et al. (1954) discovered a similar (or identical) protein and carried 

 the matter a step further by crystallizing it. This crystallizable pro- 

 tein behaves in the same way as does Markham and Smith's (1949) 

 material, i.e., it is immunologically identical with the virus, it is non- 

 infective and it is free of RNA. 



Whether these abnormal proteins present in virus-infected plants 

 are virus precursors, intermediary stages in virus production, or 

 by-products of the virus is still undecided. The main point is that in 

 contrast with the complete virus they contain no RNA and are 

 never infective. 



Recent work on the structure of plant viruses (especially tobacco 

 mosaic virus) by crystallographic and electron microscopy methods 

 (Crick and Watson, 1956; Hart, 1955; Schramm and Zillig, 1955; 

 Zillig et al., 1955; Fraenkel-Conrat and Williams, 1955) has con- 

 siderably clarified the relationships existing between RNA and pro- 

 tein. Observations made by Schramm and Zillig (1955) on tobacco 

 mosaic virus treated with sodium hydroxide have shown that the 

 protein of the virus has a molecular weight of 17,000; on reacidifi- 



