90 D. SHUGAR 



small extent Diplococcus pneumoniae transforming DNA (R. Latarjet and N. Rebey- 

 rotte, cited in Jagger 181 ). Pakula (personal communication) confirms this and also 

 finds only slight photoreactivation for streptococcal transforming DNA under 

 conditions where H. influenzae DNA exhibits photoreactivation; in fact, it was found 

 that the DNA of the coli extracts inhibits competitively the pneumococcal and strep- 

 tococcal DNA's. A yeast extract, on the other hand, exhibits the same reactivating 

 ability as an E. coli extract on H. influenzae DNA. 183 



The above system has been applied in an ingenious manner to demon- 

 strate ultraviolet damage, as well as photoreversal of such damage, in DNA 

 which exhibits no biological transforming activity. 183, 184 The technique is 

 based on the fact that photoreversal of irradiated transforming DNA is 

 inhibited in the presence of irradiated, but not nonirradiated, nontrans- 

 forming DNA. If, however, ultraviolet-inactivated nontransforming DNA 

 is first subjected to photoreactivation, it then no longer inhibits photo- 

 reactivation of irradiated transforming DNA. 



It is worth noting that the maximum degree of photoreactivation 

 achieved, about 50%, is approximately equal to the maximum degree of 

 thermal reactivation obtained with various nucleotide chains, using spectral 

 changes as the criterion for reversibility. 



VIII. Radiation Receptors in Living Organisms 



Although we have been dealing in this chapter almost exclusively with 

 the effects of irradiation at one wavelength, 253.7 m/x, the application of 

 different wavelengths to the same system, coupled with a knowledge of the 

 absorption spectra of the various components of the system, has proved 

 an extremely useful tool in gaining additional information about the mech- 

 anism of action of radiation and, in particular, regarding the component(s) 

 of the system responsible for the initial absorption process leading to an 

 observed physicochemical or biological effect. The theoretical basis of ac- 

 tion spectroscopy has been described by Loofbourow 159 and Blum 185 and 

 the subject has recently been comprehensively reviewed. 165 



The correspondence between action spectrum and absorption spectrum 

 for a given substance is based, among others, on the assumption that 

 quantum yields are independent of wavelength. It is consequently neces- 

 sary to emphasize that for nucleic acids and their derivatives there is very 

 little information available regarding the dependence of <f> on wavelength. 

 An action spectrum has been obtained for the polymerization or cross- 

 linking of dry DNA films and it corresponds reasonably well with the 



183 C. S. Rupert, Federation Proc. 17, 301 (1958). 



184 C. S. Rupert and R. M. Herriot, 7th Intern. Congr. Microbiol., Stockholm, 1958 

 Paper No. 4s (1958). 



186 H. Blum, in "Biophysical Research Methods" (F. M. Uber, ed.), Chapter 13. 

 Interscience. New York, 1950. 



