146 C. A. KNIGHT 



B. Nucleic Adds 



All plant viruses so far examined have been found to contain ribonucleic 

 acid (RNA). As shown in Table IV, the amount of RNA in different viruses 



TABLE IV 



Approximate Quantity of Ribonucleic Acid in Some 

 PuKiFiED Plant Vmus Pkepaeations 



Virus Per cent. RNA References 



Ross (1941) 

 Bawden et al. (1951) 

 Bawden and Pirie (1938) 

 Miller and Price (1946) 

 Knight and Stanley (1941) 

 Black and Knight (1953) 

 Bawden (1950) 

 Steere (1956) 



de Fremery and Knight (1955) 

 Markham (1953) 



** Recent evidence indicates that the precise value is probably closer to 5 % than to 

 6 (Knight and Woody, 1958). 



ranges from about 5-6 % for TMV and potato virus X to about 35 % for 

 tobacco ringspot and turnip yellow mosaic viruses. Whatever the amoimt of 

 RNA, it seems to be characteristic for a group of viruses, the variant strains; 

 at least, with many strains of TMV and three of BSV it has not been possible 

 to detect significant differences in nucleic acid content among strains of the 

 same virus. The proportion of RNA in a virus thus becomes a chemical 

 criterion of strain relationship, with the reservation that alone this is insufi&- 

 cient evidence in view of the fact that some unrelated viruses have been 

 observed to have comparable amounts of RNA. 



1. Compositions of Strain Nucleic Acids 



a. Strains of TMV and of Bushy Stunt Virus. Analyses of nucleic acids for 

 constituents are usually preceded by application of acid or alkaline hydro- 

 lytic procedures. The best results can be expected when the procedures are 

 applied to isolated nucleic acids; however, the analytical methods have been 

 successfully used on whole viruses and the protein components seem to cause 

 Httle or no difficulty, especially in those cases where the acid is 10 % nucleic 

 or more. The methods for analyzing nucleic acids are weU described by 

 Markham and Smith (1954) andin Chargaff and Davidson's monograph (1955). 



