102 D. SHUGAR 



case of thymine, were postulated as being the cis and trans isomers of 4- 

 hydroxy-5-hydroperoxythymine 233 (and therefore involving saturation of 

 the 4 , 5 double bond as for photoproducts of many pyrimidines) have now 

 been shown to be so by direct chemical synthesis. 234 These peroxides may 

 be of some significance in the photochemistry of nucleic acids because of 

 the observation 231 that they are also formed by ultraviolet irradiation of 

 nucleic acids and their derivatives in an oxygen atmosphere. In view of the 

 frequent references in this text to the effects of unfiltered radiation, it 

 becomes desirable to delineate the upper wavelength limit for such peroxide 

 formation which probably is below 210 nux. 



The value of simultaneous investigations with ultraviolet and ionizing 

 radiations (Section I) is reflected in the increasing number of publications 

 devoted to such studies; particular interest attaches to those of Elkind 

 and Sutton 235 wherein evidence is presented for the existence, in dividing 

 yeast cells, of an overlap in sites sensitive to X-rays, ultraviolet, PR and, 

 most important of all, ultraviolet reactivation of, or protection against, 

 X-ray lethality. It is difficult to avoid the temptation to explain the latter 

 effect by the formation, by ultraviolet light, of 4-hydroxy-5-hydropyrimi- 

 dine derivatives which would subsequently not form hydroperoxides upon 

 X-irradiation (see above). 



Increasing attention is being devoted to the origin of the complex, or 

 multicomponent, nature of the inactivation curves for transforming DNA 

 (Section VI,2,fe) by both ultraviolet and ionizing radiations and special 

 attention is drawn to the comprehensive papers on this subject by Latarjet 

 et aL 236 and Lerman and Tolmach. 237 



Irradiation of air-dried DNA films (see Section VI, 1) at 253.7 m/j has 

 been found to result in formation of a photoproduct which forms fibers 

 insoluble in water or salt solution and resistant to the action of deoxyribo- 

 nuclease. 238 The irradiated films exhibit a considerable increase in specificity 

 for methyl green. Staining techniques with labeled dyes 239 were therefore 

 used for following the effects of irradiation on films of a variety of natural 

 and synthetic polynucleotide chains, the results indicating that the resultant 

 cross-linking is rather an ordered, than a haphazard, process involving the 

 pyrimidine rings. 238 No similar effect could be observed with 7-radiation 

 although deuteron irradiation had previously been shown to induce slight 



233 G. Scholes and J. Weiss, Radiation Research, Suppl. 1, 177 (1959). 



234 B. Ekert and R. Monier, Nature 184, B.A. 58 (1959). 



235 M. M. Elkind and H. Sutton, Radiation Research 10, 283, 296 (1959). 



236 R. Latarjet, H. Ephrussi-Taylor, and N. Rebeyrotte, Radiation Research, 

 Suppl. 1, 417 (1959). 



237 L. S. Lerman and L. J. Tolmach, Biochim. et Biophys. Acta 33, 371 (1959). 



238 D. Shugar and J. Baranowska, Nature, 135, 33 (1960); J. Baranowska and D. Shu- 

 gar, Acta Biochim. Polon., in press (1960). 



