MUTAGENESIS 91 



bromoiiracil at all, whether it is in one strand, or whether there is 

 bromouraeil in both strands of DNA. But if one could test whether 

 there would be more mutants from DNA of the third peak than of the 

 first peak, this would answer your question. That would be a direct 

 experiment. 



Goldstein: When you have DNA which has incorporated bromouraeil 

 very heavily in place of thymine, that incorporated bromouraeil must 

 pair adenine against it quite normally. The evidence for that is clear 

 ■ — otherwise you would have only dead organisms. In other words, 

 DNA can have half or more of its thymine replaced by bromouraeil, 

 and yet it is still a functional DNA. The proportion of mutant genes 

 to total genes is extremely small. This suggests to me that the mistakes 

 are made subsequent to the incorporation. 



Auerbach: How does one explain the fact that Rose Litman found 

 that the bromouraeil could be counteracted by serine in the medium? 



Freese: I think the media are so complicated that we can't be sure. 



Auerbach: Would smaller amounts of bromouraeil be incorporated, 

 or would something in the metabolism of the bacterium counteract its 

 effect in such medium? 



Freese: The sulfanilamide medium which she used is so complex 

 that I would not draw any conclusion about the detailed manner in 

 which the mutations arose. 



Auerbach: So one should conclude that the most plausible explana- 

 tion is that bromouraeil acts by being incorporated, but it is not 

 proved that it acts so. Now, how is 2-aminopurine incorporated? 



Freese: We have measured in collaboration with Dr. Gottschling 

 the incorporation of 2-aminopurine in phage T4 DNA and found that 

 about one in 3000 adenine bases is replaced by — well, I shouldn't say 

 that. I should say that the amount of incorporation of 2-aminopurine 

 is 1/3000 of that of adenine into the DNA. 



Goodgal: You have your nearest-neighbor analysis? 



Freese: Oh, no. 



Zamenhof: What is the possible error in determination of that small 

 amount? 



Freese: This has been done with radioactive purine. We have 

 dialyzed the DNA for seven days and then digested it and found the 

 aminopurine, whatever that treatment means. I cannot exclude that 

 an amino-purine base has occasionally been attached to the DNA 

 molecule without being incorporated, but that does not seem very 

 likely. 



Auerbach: Then how do you explain that aminopurine is so ex- 



