240 THE EFFECT OF IONIZING RADIATIONS 



sulfhydryl groups as the embryo is allowed to metabolize. This, however, shows 

 up 10-18 days after irradiation if the eggs are kept at room temperature. 



Plough: 



I have been very much impressed by Barron's interesting and suggestive 

 analysis and his clear evidence that an important effect of radiation occurs in the 

 oxidation-reduction systems. I am, however, a little disturbed by the fact that 

 all the evidence he has cited for bacteria has to do with very low radiation 

 dosages — as I recall, about 10-50 r. From these results he concludes that radia- 

 tion has no effects on nucleoproteins. I feel that I must point out that all the 

 data bearing on changes in nucleoproteins, that is, gene mutations, come from 

 the utilization of very much higher dosages. For instance, in our rather ex- 

 tensive work at Amherst on what appear to be gene mutations in the bacterium 

 Salmonella typhimurium we get no permanent effects until we reach dosages of 

 25,000-100,000 r. In this range we get many auxotrophic (or nutrilite-requir- 

 ing) mutants, and they are roughly in proportion to the dosage. Below 50,000 r 

 the most frequent mutant is a cysteine-requiring strain. Thus it does appear 

 that effects on nucleoproteins do occur, and they have to do with reducing 

 activities. 



Barron : 



I should like to know the name of the bacteria Plough worked with. 



Plough: 



Work with Salmonella shows that gene changes appear with higher doses than 

 those used by Barron. This is a common finding. 



Barron : 



The statement on nucleoproteins was made to raise comment. Direct effects 

 on the nucleoprotein cannot be ruled out. Denaturation and precipitation may 

 also occur. 



