542 RADIATION BIOLOGY 



prodigiosum, mutations resulting in stunted colonies are produced by the 

 action of visible light on organisms stained with erythrosine. 



Contrary to the one-hit frequency-dosage curve which Kaplan (1948b) 

 had found for ultraviolet with Bacterium prodigiosuni, that for visible 

 light was found to be a two-hit curve, the induced frequency being 

 proportional to the square of the dose. Since at the same time the muta- 

 tion frequency was largely independent of timing and (for lower concen- 

 trations) linearly proportional to the erythrosine concentration, Kaplan 

 concluded that the mutations are caused by the accumulation of two 

 quanta on the same individual dye molecule and that the effects of any 

 given quantum absorption are retained for more than 6 hours. It was 

 inferred that the reason two quanta are needed is because of the relatively 

 low energy of the visible light here in question, about 2 ev, w^hereas each 

 effective ultraviolet quantum contains from 5 to 6 ev. In addition, he 

 found that at exceedingly high concentrations, erythrosine has a mutagenic 

 effect even in the absence of light. In similar work with fiuorescein- 

 stained E. coli of a special histidineless stock, Kaplan (1950a) found the 

 frequency of the mutations under observation (back mutations to histi- 

 dine independence) to follow a multihit curve similar to that found with 

 ultraviolet in this stock (see p. 537). In Drosophila, Liiers and Borch- 

 ert (1949) have reported evidence, as yet inconclusive, indicating that 

 visible light produces mutations in flies which contain acridine orange. 



The results with visible light must involve a kind of transfer of energy, 

 or reaction chain, leading from the material in which the original absorp- 

 tion and activation occurred to the genetic material which undergoes the 

 mutation. This in itself suggests that with ultraviolet also such a trans- 

 mission of effects may take place. This subject is considered on p. 

 550. In the meantime it may be noted that, if such a transmission 

 exists, then the mere fact that nucleic acid usually happens to be the main 

 site of absorption of ultraviolet does not carry with it the implication that 

 an activation occurring in this substance is more likely to be mutagenic 

 than an activation produced in some other material. This consideration 

 in turn supports the view (see p. 539) that, contrary to the contention 

 of some authors (e.g., Lea, 1946), the mutagenic efficiency of X-ray- 

 induced excitations cannot safely be estimated on the basis of data con- 

 cerning those induced by ultraviolet. 



12. REPAIR AND ENHANCEMENT IN ULTRAVIOLET MUTAGENESIS 



In connection with the dosage studies mention has been made of the 

 possible existence of a reparative process, which tends to prevent 

 potentially mutagenic effects of ultraviolet from proceeding to effectuate 

 permanent point mutations. The first convincing evidence of such a 

 process was obtained in studies concerned with the possible influence of 



