X. TMV STUDIES IN GENETIC CODING 489 



yama and Fracnkel-Conrat, 1961; Sugiyama et al., 1962; Whitfeld, 

 1962; Fraenkcl-Conrat and Singer, 1962). 



The other approach is a quantitative study of the cUgestion products 

 obtained from RNA with specific enzymes. Structural work along those 

 Hues may be facilitated by recent improvements in oligonucleotide frac- 

 tionation methods, such as the 2-dimensional mapping procedure of 

 Rushizky and Knight (1960), and the column chromatographic method 

 of Staehelin et al. (1959). 



IV. Viral Infectivity 



A. CHEMICAL BASIS OF INFECTIVITY 



It was reported in 1955 that TMV-RNA by itself had little biological 

 activity, but that the coaggregation of native TMV protein with TMV- 

 RNA prepared at neutrality yielded particles which were several hundred 

 times more infective than either component (Fraenkel-Conrat and 

 Williams, 1955). At that time it seemed that infectivity was a property 

 only of the complete virus, be it isolated from a plant, or reconstituted 

 in vitro from its components. Further studies, however, revealed that the 

 low infectivity found in RNA preparations represented an intrinsic 

 property of the RNA itself, and that this infectivity was affected by the 

 protein coating only in a Cjuantitative sense (Fraenkel-Conrat, 1956; 

 Gierer and Schramm, 1956). This conclusion has been confirmed by 

 others and has been supported by independent observations with other 

 viruses including animal viruses, as reviewed by Colter (1958) and 

 Fraenkel-Conrat (1962). 



The main pieces of evidence for the intrinsic infectivity of the RNA 

 were (1) that traces of ribonuclease destroyed the infectivity of the 

 RNA, while proteases did not, (3) that the infectivity was sedimentable 

 together with the RNA, and not at the high rate characteristic for the 

 virus and (3) that a variety of chemical and serological tests failed to 

 detect in the RNA significant amounts of contamination by virus protein 

 or peptide. In addition, electron microscopy and other physical methods 

 have been employed and all observations support the conclusion that the 

 RNA carries the infectivity. 



We may now raise the question of why the RNA has only about 

 0.1% of the infectivity expected from the activity of TMV. Similarly 

 low infectivity is observed in RNA preparations from different sources 

 and prepared by various methods. This low infectivity may be presumed 

 to be due to the RNA being damaged in the course of purification. 

 However, the fact that the low activity of the RNA is greatly increased 

 upon reconstitution and then approaches that of the original virus 



