104 MUTATIONS 



Zamenhof: Yes. 



Auerbach: How can you reverse it, then? 



Zamenhof: In certain cases, nucleoside triphosphate may again re- 

 place deoxyribose triphosphate. In other cases mutations may be non- 

 reversible. We have several "deletions" produced by heat; these dele- 

 tions may or may not be the result of depurination. Of course, you 

 never can tell about the nature of a "deletion." "Deletion" usually 

 means that the mutant never reverts. But this could be because the in- 

 jury is very large or because the injury is too severe, as when the sugar 

 is destroyed; in this last case, the DNA could never hold a purine again. 



Auerbach: But if the purine is out, you couldn't put it in again, 

 could you? You couldn't reverse the mutation. 



Freese: I could even imagine that in the resting DNA, already the 

 empty place is filled by something. We don't know of any enzymatic 

 mechanism at the present time which would do that, but it is not im- 

 possible; you have the nucleotide phosphate here, and it is not im- 

 possible that there is some enzyme which hooks on the base at this 

 place before the DNA duplicates. We don't know. 



Benzer: Green and Krieg's experiments show that what you get is an 

 activated phage, which is mutation-prone and continues to throw off 

 mutants during replication (31). 



Novick: Excuse me, but before you go ahead, would you explain 

 your interpretation? What would you conclude from this? 



Freese : I would conclude — and we have worked with other mutagens 

 which agree with this picture — that these aminopurine induced mu- 

 tants which are especially highly inducible to revert by EES have a 

 guanine-cytosine pair, and most bromouracil induced mutants have an 

 adenine-thymine pair at the mutant site. This we conclude from the 

 already mentioned chemical observation that guanine is attacked 

 preferentially. 



Lederberg: There is one point in the chemistry on which I am not 

 clear. You referred to the specificity of this agent in removing guanine. 

 Was there a comparable specificity in the ethylation itself? You re- 

 ferred to other products of ethylation which are not unstable, which 

 don't give you — 



Freese: Well, here, I can only refer to the experiments by Zamenhof 

 (60) and Lawley (43), who have shown, with methylating agents, that 

 the preferential effect on nucleic acid bases is the one of alkylation 

 of the 7 group of guanine. There is also an alkylation of the 1 and the 

 2-position of adenine, and probably the 1-position of cytosine, but 

 this is much less frequent. We have two more arguments which indi- 



