30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 97 



(a) Most photoreactivable phenomena include reproduction, mutation or trans- 

 formation, which involve the nucleus and therefore DNA-protein; there are also two 

 authentic examples of cytoplasmic PR. In the case of bacteriophage there can be no 

 doubt that PR involves DNA-protein and the evidence appears to indicate that the 

 PR site is DNA. Insofar as plant viruses are concerned, any previous doubts 181 as to 

 their general susceptibility to PR has now been resolved by the demonstration that 

 infectious RNA from TMV is photoreactivable. 170 Coupled with the demonstration 

 of PR in transforming DNA, reasonably good evidence exists that the site of PR in 

 living organisms is DXA and/or RNA. 



(6) Ultraviolet damage in vivo may be restored or repaired by agents other than vis- 

 ible light and, of these, the one that most closely resembles PR is heat, or thermal 

 restoration (TR). 



(c) PR is not a 100% phenomenon and is usually of the order of 50% or less. 



id) The available evidence implies that PR operates via reversal of damage caused 

 by ultraviolet and not through the formation of new metabolic pathways. 



Facts (a) and (d) have stimulated a number of authors to propose gen- 

 eral theoretical models to account for PR on the basis of such physical 

 principles as electron capture in crystals (Herschel effect), the formation of 

 metastable states, internal conversion, etc., 6 ' 210 all of which are conceivable 

 in rigid macromolecules, such as DNA, where conditions similar to those 

 in the solid state may possibly exist. 



The demonstration, on the other hand, that the reversible photolysis of 

 pyrimidine nucleotides is likewise exhibited by these constituents in nu- 

 cleotide chains, provides an experimental model which is reasonably con- 

 sistent with all the above facts, if due consideration is given to the possi- 

 bility that TR and PR merely represent two different sources of energy for 

 the reactivation process. It must be borne in mind that TR has received 

 considerably less attention, experimentally, than PR; it is also less specific. 



It would obviously be desirable to learn something about the specificity 

 requirements of the light-activated enzymic mechanism responsible for PR 

 in transforming DNA; and we have made several attempts to observe PR 

 for the reversible photoproducts of dimethyluracil and uridylic acid with 

 illuminated E. coli B extracts (under conditions where H. influenzae trans- 

 forming DNA exhibits PR) but with negative results (Janion and Shugar, 

 unpublished). It is, however, quite possible that the specificity require- 

 ments of this enzyme include the presence of an internucleotide linkage. 



A number of attempts have been made to reverse the effects of ultraviolet radia- 

 tion on model substances by exposure to light of longer wavelength, which we shall 

 now summarize. 



Kita et al. 2U report that the decrease in absorption of ATP resulting from exposure 



210 (a) G. Stein, J. chim. phys. 51, 133 (1954); (b) J. Duchesne and J. Garsou, ibid. 

 54, 789 (1957); (c) J. Duchesne and B. Rosen, ibid. 56, 76 (1959). 



211 H. Kita, H. Maede, B. Hanazaki, F. Shimizu, and H. Fujita, Bull. Tokyo Med. 

 Dental Univ. I, 37 (1954). 



