476 HADIATIOX MIOLOGY 



and B r; that it is absent after X-ray irradiation; and that it takes place in 

 H. nrnjathcrium 899, which, like E. roli K12 is lysogenic, whereas E. 

 coli B and B/r arc not so. Therefore some relation with lysogenicity 

 is indicated. 



The catalase eiVect seems to \)v. an important feature; it is piobahiy 

 not identical with photoreactivation hut related to it. 



Johnson d (il. (19r)()) have found that oxygen is not required for bac- 

 terial photoreactivation. 



5-7. ACTION OF PIIOTOIUOACTIVATION ON THE INDTCTION OF 

 MUTATION BY ULTRAVIOLET IN liACTEUIA 



This problem is of great importance for understanding the relation 

 between inactivation and induction of mutations. Ob.servations have 

 been carried out mainly on the induction of the mutation to resistance to 

 bacteriophages in E. coli B/r (Kelner, 19-19b, c; 1950a; Novick and Szilard, 

 1949). Two different technicjues have been used for determining the 

 extent of the mutagenic action and the effect of photoreactivation. 

 Kelner has determined the number of mutations present under various 

 experimental conditions according to the spray technicjue of Demerec 

 (1946). Novick and Szilard have determined the number of mutants 

 present in a culture arising from a treated bacterial suspension after the 

 bacteria in the sample have been allowed to undergo on the average about 

 ten divisions. With regard to this method, it must be observed that 

 bacteria treated with ultraviolet show a variable lag period before starting 

 to divide, and that mutations induced by ultraviolet become phenotypic 

 after an additional lag (Newcombe and Scott, 1949); the fraction of 

 mutants in the population after growth might therefore be affected by 

 variations in the lag values. 



Kelner studied the behavior of zero-point mutations (mutations arising 

 before any division of the ultraviolet-treated cells has taken place) and of 

 delayed mutations (manifested after several cell generations). The zero- 

 point mutations appear to be completely suppressed if the bacteria after 

 the ultraviolet treatment are exposed to the photoreactivating light; the 

 fraction of delayed mutations, on the contrary, is little or not at all 

 affected by the light. 



Novick and Szilard found that the fraction of mutants in the total popu- 

 lation is considerably decreased by the action of the light, and that the 

 fraction of mutants observable after photoreactivation is comparable with 

 the amount o})tainable with a lower ultraviolet dose in the absence of 

 photoreactivation (Fig. 12-8). Induction of mutations by ultraviolet 

 would therefore be affected by photoreactivation in a similar way as the 

 reactivation, with a constant ultraviolet dose reduction produced by light. 



The difference in the results obtained l)y Kelner and by Novick and 

 Szilard cannot be accounted for by the difference in the method used, since 



