30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 95 



that U2 and RNA2 are equally sensitive at 254 mix. 167 Since the difference 

 in sensitivities between Ul and U2 at 254 m/x had previously been shown 

 to be due to protection of the RNA of the former by its protein component, 

 it was concluded that the equal sensitivities of both strains at 226 m/z 

 may be ascribed to energy absorption by the protein components alone. 

 While issue may be taken with the conclusion that inactivation of virus 

 infect ivity at 226 mix is due entirely to protein absorption and that at 254 

 m/x entirely to nucleic acid, this method of approach is of great value, 

 particularly as infectious nucleic acids from other viruses are becoming 

 available for such studies. 



Probably the most striking illustration of the role of nucleic acid-protein 

 interaction in virus activity is the demonstration by Bawden and Klecz- 

 kowski 170 that infectious RNA from TMV may be photoreactivated. 

 Previous work by the same authors 204 had shown that a variety of plant 

 viruses exhibited photoreactivation, several TMV strains being the sole 

 exceptions. 



It has been variously shown that the sensitivity of a virus bears little, 

 if any, relation to its nucleic acid content and it now is apparent that this 

 is not necessarily due to possible differences in the structure of the nucleic 

 acid components alone, but to the structural integrity of the virus as a 

 whole. 



When we attempt to correlate physicochemical changes with loss of 

 infectivity for a given virus as a result of irradiation, we encounter a situa- 

 tion similar to that for transforming DNA and infectious RNA. Com- 

 pletely inactivated TMV exhibits the same X-ray pattern, isoelectric point, 

 optical rotation, molecular weight, and ability to crystallize, although the 

 crystals are slightly opalescent; the appearance of the inactivated virus 

 under the electron microscope is unchanged, 98, 205 and its absorption spec- 

 trum is unaffected. It follows that inactivation must result from highly 

 localized injury, a conclusion also derived from studies of "cross-reactiva- 

 tion" in bacteriophage. 206 



The original observation of Stanley 205 that ultraviolet-inactivated TMV 

 retains the serological properties of the original virus first pointed to the 

 now generally recognized feasibility of separating the infective from the 

 antigenic properties of a virus by irradiation. The practical application of 

 irradiation to the production of vaccines has now been investigated for a 

 number of viruses and in at least some instances the products obtained 

 appear to be entirely satisfactory . 207a - d Ultraviolet irradiation has also been 



204 F. C. Bawden and A. Kleczkowski, J. Gen. Microbiol. 8, 145 (1953); 13, 370 (1955). 



205 W. M. Stanley, Science 83, 626 (1936). 



206 G. S. Stent and C. R. Fuerst, Advances in Biol, and Med. Phys. 7, in press (1959). 



207 (a) S. O. Levinson, A. Milzer, H. J. Shaughnessy, J. L. Neal, and F. Oppenhei- 

 mer, J. Am. Med. Assoc. 125, 531 (1944); J. Immunol. 50, 317 (1945); (b) 



