28 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



was reduced by 50%. When the virus was collected and repeatedly 

 crystallized, it was found that it had incorporated the labeled 

 thiouracil in its RNA moiety only, apparently in the form of 

 thiouridylic acid (Jeener, 1957). The correctness of these findings 

 has been confirmed by Matthews (1956) and by Mandel et al. (1957). 



Very similar findings have been reported by Matthews (1954) in 

 the case of 8-azaguanine. It is perfectly clear from all these experi- 

 ments that the infectivity of tobacco mosaic virus is closely linked 

 with the integrity of its RNA component. Alterations of the latter 

 by the introduction of abnormal bases in its molecule lead to the 

 loss of infective power, i.e. the nucleoprotein particle cannot be 

 synthesized any more. 



Since the presence of normal RNA is required, in addition to the 

 presence of the non-infectious protein, to endow the tobacco mosaic 

 virus with the capacity of synthesis, one might hope that by mixing 

 the RNA and protein constituents partial resynthesis of the virulent 

 particle might be achieved. Remarkable success came when Fraen- 

 kel-Conrat and Williams (1955) separated the virus protein by a 

 sodium hydroxide treatment, and the virus RNA with a detergent. 

 Neither of these two components was infectious as such ; but on 

 mixing together the RNA and the protein components obtained by 

 this method, active, infectious particles could be reconstituted. 

 Tobacco mosaic virus *'resynthesized" from its RNA and protein 

 constituents shows the typical rod-like appearance under the elec- 

 tron microscope. In order to obtain successful results, the tobacco 

 mosaic virus RNA must be isolated in its native form. It loses its 

 activity on treatment with ribonuclease and it cannot be replaced 

 by RNA of turnip mosaic virus or by DNA. Synthetic polymers of 

 nucleotides, prepared according to the method of Grunberg- 

 Manago et al. (1956), when added to the virus protein, can recon- 

 stitute rods; but the latter are not virulent (Hart and Smith, 1956). 



Considered together, all these experiments demonstrate that the 

 RNA molecule must remain intact for virus synthesis. But, although 

 we know that the protein part alone is non-infectious, the experi- 

 ments do not prove that RNA alone is responsible for virus multi- 

 plication. Such a proof has been adduced by a most remarkable 



