30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 99 



5-hydroxymethylcytidylic acid. 215 If these derivatives fail to exhibit re- 

 versible photolysis, serious doubts exist as to the participation of pyrimidine 

 derivatives in PR. Sinsheimer 215h mentions that 5-hydroxymethylcytidylic 

 acid and glucosylated 5-hydroxymethylcytidylic acid differ notably in 

 sensitivity and nature of response to ultraviolet, but gives no details. The 

 significance of a marked difference in behavior of these two substances may, 

 however, be appreciated when it is realized that it may provide an impor- 

 tant clue to the site of action of radiation in these viruses, as well as to the 

 mechanism involved. 



If, now, PR is due to reversible photolysis of pyrimidine rings, the modi- 

 fications in biological properties of an irradiated single-stranded nucleic 

 acid chain would result from (a) a "change" in base sequences, and (b) 

 changes in secondary structure due to modifications in intrachain hydrogen 

 bonds and to weakening of the interaction between adjacent rings, at those 

 sites where a pyrimidine ring has been transformed to the corresponding 

 4-hydroxy-5-hydro derivative. For a double-stranded chain we should also 

 expect some modification in the number of hydrogen bonds between base 

 pairs involving those pyrimidine rings which have reacted photochemically. 

 It is assumed that internucleotide bonds are unaffected, an assumption 

 which is reasonable if the radiation doses have not exceeded those which 

 are biologically effective. 



It is consequently of interest that 90 % ultraviolet inactivation of trans- 

 forming DNA leads to a detectable drop in denaturation temperature or of 

 the transition profile (i.e. of the mean temperature for the thermally in- 

 duced transition of the DXA chain from a helix to a random coil). 216 The 

 fact that the decrease is small (1.5°C.) indeed suggests that only a few 

 hydrogen bonds were either broken or weakened by the irradiation, as 

 would be expected, so that reformation of such bonds may be readily 

 visualized at those points where the original pyrimidine ring is subsequently 

 restored by PR or TR. It might be argued that a few hydrogen bonds 

 could be broken by the absorbed energy without photolysis of pyrimidine 

 rings, but in such an event one might expect spontaneous reformation of 

 these bonds. 



Extension of the above observation of Marmur and Doty 216 to trans- 

 forming DNA containing several markers with different radiation sensi- 

 tivities (Section VI, 2, b) might provide valuable information regarding the 

 relationship between biological activity and some types of base-pair se- 

 quences. 



215 (a) E. Volkin, J. Am. Chen. Soc. 76, 5892 (1954); (b) R. L. Sinsheimer, Science 

 120, 551 (1954); (c) S. S. Cohen, in "The Chemical Basis of Heredity" (W. D. 

 McElroy and D. Glass, eds.). Johns Hopkins, Baltimore, 1957; (d) M. R. Loeb and 

 S. S. Cohen, J. Biol. Chem. 234, 364 (1959). 



216 J. Marmur and P. Doty, Nature 183, 1427 (1959). 



