256 FACTORS INFLUENCING CELL RADIOSENSITIVITY 



The following are the results of a typical experiment : 



Ki2 irradiated with 1200 ergs/mm" (X 2537 A) 



Number of irradiated bacteria plated per Petri dish: 4 X 10^ 



Number of colonies after 36-hr incubation: 



Irradiated control (dark) 22 



Irradiated control (+ visible light) 220 



Irradiated control (+ catalase dark) 35 

 Irradiated control ( + catalase, same dose + visible 



light) 2500 



(Sometimes catalase alone would give 1000 colonies.) 



4. This phenomenon seems to require much more stringent conditions 

 than photoreactivation. For example, we were not able to reproduce it 

 in bacterium B under the conditions used with K12, although these con- 

 ditions allowed B to undergo strong photoreactivation and although this 

 bacterium had shown increased resistance to ultraviolet radiation when 

 grown in the presence of catalase. 



5. This recovery takes place in K12 even when catalase is added some 

 time after irradiation, the length of the time depending on the tem- 

 perature. In cultures kept at 37°, reactivation is still possible when 

 catalase is added 2 hr after irradiation. It is thus evident that the 

 action of the enzyme involves relatively stable compounds. 



6. No reactivation was obtained after x-irradiation. Such facts 

 might throw some light upon the origin of the radioresistance displayed 

 as a hereditary character by some mutant strains (31). 



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