MUTAGENESIS 103 



incorporated preferentially across the gap are the pyrimidines, because 

 we apparently get more of the above mentioned mutagenic transitions 

 than the changes in which a purine is replaced by a pyrimidine, or 

 vice versa. 



Auerbach: I'm sorry, but what is the evidence that you got these 

 changes? I have forgotten. 



Freese: Because 70 per cent of the mutations we get in this case are 

 inducible to revert by the base analogues bromouracil and 2- 

 aminopurine. 



Glass: I don't quite understand. Is the change on the charge of the 

 nitrogen in position nine going to produce depurination? What makes 

 the depurination take place? 



Freese: Actually, you have to imagine that the positive charge is 

 distributed over the imidazole ring, and whenever you have such a posi- 

 tive charge, these groups are more labile. That is just a chemical fact. 

 It corresponds to our observation that the guanine goes off. 



Glass: They fail to become attached to the deoxyribose? 



Freese: No, this is DNA. You can take free DNA, which has the 

 guanine in the DNA ; treat it with diethyl sulfate, and then the guanine 

 is removed from the DNA. 



Neel: And you recover guanine in the experimental system? 



Freese: You recover guanine in the dialysate. 



Auerbach: And this is due to the bond with the sugar being broken? 



Glass: Yes, that's what I meant. 



Atwood: A few months ago, Krieg and Green had some preliminary 

 experiments which suggested that mutations induced by ethylmethane 

 sulfonate in phage were not reversible by that same agent. That is 

 predicted from your model, is it not? 



Freese: Right. We have also done experiments on the induction of 

 reversions by ethylethane sulfonate, and we found that certain 

 mutants which had been induced by aminopurine are much more in- 

 ducible to revert than most of the mutants induced by bromouracil. 



Zamenhoj: Ernst, as you know. Dr. Greer in our laboratory found 

 that heat depurinates under mutagenic conditions (32) ; would you 

 agree that it is not completely impossible that the depurinated locus 

 always reproduces as depurinated rather than picks up a wrong base? 

 I think that Kornberg is now investigating whether or not deoxyribose 

 triphosphate might participate rather than nucleoside triphosphate, 

 that is, whether the former can stay naked, so to speak, after reproduc- 

 tion. 



Freese: To stay naked all the time? 



