DISCUSSION 295 



Plough : 



I should like to comment briefly on HoUaender's discussion of the relation of 

 oxygen tension to the production of mutations by radiation. He and others 

 have said that many other chemical processes must be taken into account in 

 connection with the problem, and I should hke to call attention to certain rele- 

 vant facts from our studies of radiation-induced auxotrophic mutations in 

 Salmonella. Just as in E. coli, we now have strains of S. typhimurium which 

 differ markedly in resistance to radiation. In strain 533 (sensitive) we can 

 isolate after exposure to a dosage of about 50,000 r about 70 per cent of bio- 

 chemical mutants — after penicillin screening — whereas with strain 519 (resist- 

 ant) only about 20 per cent of similar mutants appear after the same treatment. 

 Thus the frequency of mutations parallels the resistance to radiation. Yet the 

 optimal oxidation-reduction potential for each of these strains is the same. On 

 the other hand, preliminary tests indicate that *S. neicyort grows best at a much 

 lower oxidation potential than does S. typhimurium (strain 533), and yet the 

 frequency of radiation-induced mutations in relation to dosage is approximately 

 the same through the range of dosages tried. It would certainly appear from 

 this finding that other factors than available oxygen influence the frequency of 

 radiation-induced mutation. 



Hollaender: 



I agree with Plough that the need of the organism for oxygen is not the de- 

 ciding factor as to whether it will be radiation resistant or not. It appears from 

 our findings that oxygen is an important factor in regard to x-ray sensitivity 

 because of (a) its prevalence in the medium in which the organisms are suspended 

 during irradiation; and (b) the presence of oxygen inside the organism. Absolute 

 resistance of the organism is, of course, also influenced by many factors such as 

 inherited resistance, nutritional background, and growth phase during which 

 the organism is irradiated. 



