34. THE RIBONUCLEIC ACIDS OF VIRUSES 271 



either strain was found to 3.4 X 10~ 4 , assuming a molecular weight of 2.5 X 10 6 . 

 Since many viruses have quite different sensitivites to UV light at 254 mju, it would 

 be most interesting to see if infectious RNA isolated from different viruses all have 

 the same quantum yield. 



The partial reactivation of the activity of UV irradiated bacterial viruses 

 can be accomplished by exposing recently infected host cells to visible 

 light. 88 It has not been possible to induce a similar photoreactivation of 

 UV irradiated TMV after inoculation on tobacco leaves. However, Bawden 

 and Kleczkowski 89 claim that phenol-extracted TMV-RNA, which is in- 

 activated by UV irradiation, can be photoreactivated by exposure to day- 

 light after inoculation on tobacco leaves. In control experiments, intact 

 virus was inactivated with UV light and RNA extracted from such inactive 

 virus. This RNA could not be photoreactivated. Thus, it may be that al- 

 terations in the protein component, or more likely in the type of bonding 

 between protein and RNA, are responsible for the UV inactivation which is 

 no longer reversible with visible light. 



Heat inactivation. The heat inactivation of TMV-RNA was studied using 

 RNA prepared by the detergent method. 90 Inactivation experiments were 

 performed at neutral pH in the temperature range 37-65°C. The inactiva- 

 tion follows first-order kinetics, as is also the case for intact virus (Fig. 7). 

 From the rate constants for inactivation at different temperatures, the heat 

 of activation (AH*) leading to loss of infectivity was calculated to be 19 

 kcal./mole. In contrast, the heat of activation for heat denaturation of 

 transforming principle is 93 kcal./mole. 91 This value, which is much higher 

 than that for TMV-RNA, may be interpreted by assuming that some 15 

 successive pairs of H bonds in the Watson-Crick DNA structure must be 

 broken before an irreversible collapse of the molecule occurs. The lower 

 value of AH* for TMV-RNA is probably based on a splitting of single phos- 

 phate-ester bonds which result in inactivation. It is much too low to be 

 explained in terms of splitting a number of H bonds as in the case of DNA. 

 This is also suggested by the negative value of the entropy of activation 

 ( — 19.5 entropy units) 90 which may also reflect the formation of a cyclic 

 phosphate triester intermediate in the activated state of the molecule. It 

 indicates that the activated molecule has less degree of freedom because 

 of the added bond between the phosphorus atoms and the 2-OH of the 

 ribose. This cyclic intermediate can be hydrolyzed easily, accompanied by 

 splitting of the polynucleotide chain. 



(2) Action of Enzymes. Enzymes which can split the polynucleotide chain 



88 R. Dulbecco, J. Bacteriol. 59, 329 (1950). 



89 F. C. Bawden, F. R. S. Kleczkowski, and A. Kleczkowski, Nature 183, 503 (1959). 



90 W. Ginoza, Nature 181, 958 (1958). 



91 S. A. Rice and P. Doty, J. Am. Chem. Soc. 79, 3937 (1957). 



