34. THE RIBONUCLEIC ACIDS OF VIRUSES 269 



splitting of the RNA molecule. The inactivation of TMV-RNA in the dry 

 state or in the frozen state follows first-order kinetics. 81 The X-ray dose for 

 37% survival (1/e dose) is the same for TMV-RNA and for TMV, amount- 

 ing to 3.0 X 10 5 r., and suggests that the X-ray inactivation of intact TMV 

 is due to alterations in the nucleic acid component. The "radiosensitive 

 molecular weight" for RNA, corresponding to a dose of 3.0 X 10 5 r. is 2.1- 

 4.0 X 10 6 . A single X-ray induced split is sufficient to inactivate the mole- 

 cule. Thus, these studies lend further support to Gierer's 11 estimate of the 

 molecular weight of biologically active TMV-RNA based on the kinetics 

 of ribonuclease degradation. The exponential inactivation over a wide range 

 of X-ray doses implies that the infect ivity is carried by particles having the 

 same molecular weight and which are not polydispersed with respect to 

 radiation sensitivity. This is in contrast to the findings with respect to the 

 X-ray sensitivity of transforming principle. 82, 83 



Action of UV irradiation on RNA. UV irradiation of virus or RNA re- 

 sults in a loss of biological activity. Whereas X-rays probably inactivate 

 virus RNA by splitting the polynucleotide chain, the mechanism of UV 

 inactivation is not yet understood. After irradiating TMV-RNA in 0.02 M 

 phosphate buffer at pH 7.3, McLaren and Takahashi 84 reported no changes 

 in the intrinsic viscosity at 6°C. The only observable change was a slight 

 reduction in optical density at 260 m/x, amounting to less than 1 %. Since 

 the inactivation seems to be a one-hit process, it was suggested that only a 

 local change in the RNA molecule is involved. 



Different strains of TMV differ markedly in sensitivity to inactivation 

 with UV light. Two strains (Ul, the common strain, and U2) were studied 

 in detail by Siegel et al. sb Upon wet irradiation the two strains differed in 

 sensitivity by a factor of 5.5 (Fig. 6). However, upon irradiation of dried 

 preparations, the same sensitivity was found for both strains and, indeed, 

 the same as the sensitivity of RNA extracted from either strain. 86 The 

 detergent extraction of RNA was employed and irradiation performed at 

 254 m/z. The sensitivity of dried virus, as well as that of extracted RNA, 

 corresponds to the sensitivity of wet irradiated U2 virus. Thus, it appears 

 as if the protein of Ul virus has a protective action when this strain is 

 irradiated in solution. It is not yet known how this action may occur. It 

 was possible to show that strain Ul is denser than U2 by equilibrium 

 centrifugation of a mixture of the two strains in a density gradient. RNA 



81 W. Ginoza and A. Norman, Nature 179, 520 (1957). 



82 H. Ephrussi-Taylor and R. Latarjet, Biochim. et Biophys. Acta 16, 183 (1955). 



83 J. Marmur and D. J. Fluke, Arch. Biochem. Biophys. 57, 506 (1955). 



84 A. D. McLaren, W. N. Takahashi, Federation Proc. 16, 220 (1957). 



85 A. Siegel, S. G. Wildman, and W. Ginoza, Nature 178, 1117 (1956). 



86 A. Siegel, Nature 180, 1430 (1957). 



