PHOTOREACTIVATION 



477 



Kelner's data show that the effect of photoreactivation on the delay in 

 appearance of mutations is, if any, a reduction of the lag period. This 

 would affect the fraction of mutants in the method used by Novick and 

 Szilard in the direction of increasing it — in a direction, therefore, which is 

 opposite to the observed difference. 



Kelner points out that the curve giving the frequency of delayed muta- 

 tions among survivors as a function of ultraviolet dose for E. coli B/r rises 



10" 



q: 

 UJ 



a. 

 en 



5 



-.3 _ 



100 

 D , sec 



Fig. 12-8. Logarithm of number of phage-resistant mutants per 10* bacteria as a 

 function of the logarithm of the ultraviolet dose D in darkness and after maximum 

 photoreactivation for (a) phage T4, (h) phage T6, and (r), phage Tl. The bacteria 

 had passed through 10 generations in liquid culture prior to being assayed for the 

 mutants. {From Novick and Szilard, 1949.) 



sharply at low ultraviolet doses and less at high ultraviolet doses (Demerec 

 and Latarjet, 1946); therefore, if the light causes a constant reduction of 

 the ultraviolet dose also for the mutagenic effect, the relation between the 

 fraction of mutants before and after photoreactivation should vary 

 according to the ultraviolet dose used. At a low ultraviolet dose, where 

 the curve giving the fraction of mutations versus the ultraviolet dose has a 

 steep slope, the fraction of mutants should considerably decrease after 

 photoreactivation, whereas at a high dose, where the slope is less, the frac- 

 tion of mutants should be less affected. This can account for part of the 

 difference in the results obtained by Kelner and by Novick and Szilard, 



