100 D. SHUGAR 



XL Addendum 



One example has been found of thermal reactivation of a bacteriophage. 

 Partial reactivation of phage Tl, adsorbed on E. coli, was obtained by- 

 warming at 45°C. 217 Independent confirmation of this observation, as well 

 as its extension to other phages, particularly the T-even phages (see next 

 paragraph), would be extremely useful. 



An extensive investigation of the photochemistry of a variety of substi- 

 tututed pyrimidines included some preliminary observations on the photo- 

 chemical behaviour of 5-substituted cytosines and their nucleosides and 

 nucleotides, in relation to photoreactivation of the T-even phages 218 (see 

 Section X). Although no definite evidence for reversibility was obtained, 

 these derivatives proved to be relatively radiation resistant; in addition 

 the primary photoproducts are relatively stable and contain the intact 

 pyrimidine ring, indicating that irreversible damage has not necessarily 

 occurred. 



Beukers et al 2i9 - 220 report some remarkable observations on the behavior 

 of orotic acid in the presence of various gases and of thymine in the frozen 

 state (cf. Section V, 4, c). Orotic acid, e.g , if irradiated in nitrogen, exhibits 

 partial reversibility on subsequent irradiation in oxygen. The decrease in 

 absorption of an irradiated frozen solution of thymine may be almost com- 

 pletely restored by thawing under continued irradiation; the resulting 

 product differs chromatographically from thymine, but the fact remains 

 that the pyrimidine ring has been regenerated. An adequate explanation 

 for these findings requires additional data, but they emphasize once more 

 the importance (Section 11,3) of investigating the excited states of pyrimi- 

 dines by flash photolysis. 



The foregoing authors 221 have also shown that irradiation of APA leads 

 to destruction of thymine residues at a rate considerably greater than that 

 to be expected from the behavior of free thymidylic acid. A similar finding 

 has been obtained 222 by irradiation of an oligonucleotide consisting mainly 

 (90%) of thymine isolated from thymus DNA 223 but efforts to demonstrate 

 reversibility (see Section V,4,c) were unsuccessful. The observation of 

 Griffin et al.' 22i that poly ribothymidy lie acid exhibits up to 33% hyperchro- 

 micity may be related to this increased sensitivity of thymine, when 

 incorporated in a nucleotide chain, if it is recalled (Section IV,4) that poly- 



217 C. Z. Bresch, Z. Naturforsch. 5b, 420 (1950). 



218 K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon. 7, 63 (1960). 



219 R. Beukers, J. Ijlstra, and W. Berends, Rec. trav. chim. 78, 879 (1959). 



220 R. Beukers, J. Ijlstra, and W. Berends, Rec. trav. chim. 78, 883 (1959). 



221 R. Beukers, J. Ijlstra, and W. Berends, Rec. trav. chim. 78, 247 (1959). 



222 K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon. 7, No. 2-3, in press (1960). 



223 A. Adamiec and D. Shugar, Naturwissenschaflen 46, 356 (1959). 



224 B. E. Griffin, A. R. Todd, and A. Rich, Proc. Natl. Acad. Sci. U. S. 44, 1123 (1958). 



