270 



HEINZ SCHUSTER 



4 5 6 



DOSE (MIN.) 



Fig. 6. The ultraviolet sensitivity of TMV strains Ul and U2, and the infectious 

 nucleic acids derived therefrom; X = Ul nucleic acid, O = U2 nucleic acid, broken 

 line = Ul intact virus, continuous line = U2 intact virus. [A. Siegel, S. G. Wildman, 

 and W. Ginoza, Nature 178, 1117 (1956).] 



is extracted more easily from U2 than from Ul when the detergent method 

 is used. Strain U2 is more sensitive to heat denaturation than Ul. These 

 observations led to the hypothesis that the bonding or type of association 

 between protein and RNA is different for the two strains. This might lead 

 to different configurations of the RNA in the two strains, and perhaps the 

 RNA in strain Ul is in such a configuration that it can be better protected 

 from UV radiation. 



The two strains, Ul and U2, were also irradiated in solution at different wave- 

 lengths of UV light and the resulting action spectrum for inactivation was similar to 

 the absorption spectrum of RNA in the wavelength range 254-280 im«, indicating that 

 RNA is the principal chromophore responsible for the inactivation of both strains 

 in this range of wavelengths. 87 In the range 226-254 m>i, the action spectrum no longer 

 corresponds to the absorption spectrum of RNA and, further, the sensitivity of both 

 strains is the same at 226 mp, whereas at 254 and 280 nut the sensitivities differ by the 

 same factor of 5.5 mentioned above. Thus, at 226 m/x, the inactivation of both strains 

 must be due to a primary absorption of energy in the protein. The quantum yield 

 (number of molecules affected/number of quanta absorbed) of nucleic acid from 



87 A. Siegel and A. Norman, Virology 6, 725 (1958). 



