BIOSYNTHESIS OF PROTEINS 275 



the process in some way so as to give specific properties to 

 the final product of the synthesis. Hence one may see what 

 wide vistas are opened up by the study of viruses towards an 

 understanding of the significance of nucleic acid derivatives 

 in the specific synthesis of proteins. 



The tobacco mosaic virus, which has been obtained in 

 crystalline form, has been studied in detail by numerous 

 workers using the most dixerse and refined apparatus and 

 methods — by X-ray crystallographic analysis, with the elec- 

 tron microscope, with the ultracentrifuge, by the incorpora- 

 tion of labelled atoms, by chromatography, etc. (Fig. 22). 



Until very recently indeed it was held that the nucleic acid 

 in tobacco mosaic virus was exclusively RNA, but a com- 

 munication has just appeared showing that in this virus, as 

 in several other viruses, there is a small amount of DNA.^^° 

 Thus, even from this point of view, the crystalline virus does 

 not seem to be a single substance. 



As the investigations of R. Markham^" have shown, the 

 ribonucleic acids of tobacco mosaic virus, tomato bushy stunt 

 virus, turnip yellows mosaic virus, potato X virus and one of 

 the tobacco necrosis viruses are different from one another 

 in their composition and furthermore each of them differs 

 substantially from the ribonucleic acid of the respective host. 

 In the viruses ^vhich have been listed, the differences in 

 proportion of the nucleotides contained in their ribonucleic 

 acids are so great that they can serve as criteria for differen- 

 tiating between one of these viruses and another. The same 

 is true of the protein parts of the nucleoproteins of these 

 viruses. 



The amino acid composition of the protein of tobacco 

 mosaic virus is fairly accurately worked out and is similar 

 to that of a globulin. At present 18 ' common ' amino acids 

 have been obtained from it ; they are without exception 

 L-isomers.^*^ The molecular weight of tobacco mosaic virus 

 particles is very high, about 40 million, but the fundamental 

 protein molecules which take part in the structure of the 

 virus have a molecular weight of about 17,000. This may be 

 demonstrated by destroying the virus Asith ultrasonic vibra- 

 tions or detergents. Similar values for the molectilar weights 

 of the fundamental proteins have also been obtained by 



