9» D. SHUGAR 



to ultraviolet could be reversed by visible radiation; but this is not substantiated 

 in the text of their article. It would indeed be surprising if there were any effect of 

 visible light on the photoproduct of ATP, since neither one nor the other absorb in 

 the visible region of the spectrum; and this is most likely the reason for the failure 

 of Wells and Johnson 212 to find any such effect for ATP. A similar explanation applies 

 to the unsuccessful efforts of Ekert and Jagger 181 to photoreactivate, with visible 

 light, the photoproducts of dimethyhiracil, uridylic acid, and some other analogs 

 of these compounds. 



Wells 213 claims to have observed PR for DPN, but the following considerations sug- 

 gest the necessity for independent verification of this observation. The supposed PR 

 was obtained by illumination with a GE CH-4 spotlamp through a Corning 3060 filter. 

 This filter transmits only 0.5% at 365 nut and zero below this wavelength; but neither 

 DPN nor its photoproducts absorb at 365 m/x. It is therefore likely that the effect ob- 

 served resides in the crude tissue extracts used to test for DPN activity. Ekert and 

 Monier 127 attempted to duplicate Wells' experiments, using an established enzymic 

 assay procedure for DPN activity; the results were negative, as would be expected 

 from the fact that the PR wavelengths used were far to the red of the DPN absorp- 

 tion band. For this same reason their proposal that Wells' results may have been true 

 PR involving some enzyme system in the crude extracts, similar to that responsible 

 for PR in transforming DNA, is open to some doubt. 



The photoproduct of 2-methoxycytosine, which is largely reversible to the parent 

 compound by TR, exhibits an absorption band at 310 rm* and extending to 330 im». 39 

 Irradiation of this photoproduct with wavelengths just above 300 im* gave no reversal ; 

 nor has PR been found for several photoproducts, including irradiated RNA, exposed 

 to visible light in the presence of photosensitizing dyes (Wierzchowski and Shugar, 

 unpublished). 



It is clear that some of the attempts to demonstrate PR for model sub- 

 stances have ignored the first law of photochemistry, and future efforts 

 should take this into account. It would obviously be desirable to attempt 

 TR trials with infectious RNA from TMV and with transforming DNA. 

 No efforts appear to have been made to obtain TR in viruses and bacte- 

 riophage, but such experiments are undoubtedly worth trying, due con- 

 sideration being given to possible denaturation at the temperatures em- 

 ployed. 



Attention should be drawn to at least one potential argument against 

 the proposal that PR involves reversible photolysis of pyrimidine nucleo- 

 tides. The T-even phages exhibit PR but their pyrimidine components 

 include only thymine and 5-hydroxymethylcytosine. 214 It was pointed out 

 in Section V, 4, c, that not all 5-substituted pyrimidines exhibit reversi- 

 bility; but that thymine incorporated in a nucleotide chain conceivably 

 may, 154 while some evidence exists that 5-hydroxymethylcytosine does ex- 

 hibit reversibility. The question of thymine is common to all nucleic acids 

 but that of 5-hydroxymethylcytosine is unique for the T-even phages in 

 that they contain 5-hydroxymethylcytidylic acid as well as glucosylated 



212 P. H. Wells and H. Johnson, Anat. Record 117, 644 (1953). 



213 P. H. Wells, Science 124, 31 (1956). 



214 G. R. Wyatt and S. S. Cohen, Nature 170, 1072 (1952). 



