108 MUTATIONS 



with this statement bromouracil, in fact, does affect precisely the CG. 

 But it does not give you the substitution of one for the other. 



Freese: All I would say is that ethylethane sulfonate should prefer- 

 entially attack the CG base pairs. What it makes out of them, I cannot 

 predict. 



Atwood: If you think that the Green experiments are sufficiently 

 conclusive — I don't know whether they are — then you're not getting 

 transversions. 



Freese: He has not shown anything of this kind. 



Atwood: If, in so far as you would get transversions instead of 

 transitions, they would be reversible by the ethylmethane sulfonate. 



Freese: What would be reversible? 



Atwood: If you merely turn it around so that you now have a GC 

 instead of a CG, this could be reversible by another transversion. 



Freese: Right; only these changes could be reversible. But if you 

 get the change from GC to TA, for example, which is also a transver- 

 sion, it would not be reversible by EES. 



Atwood: At least if you find, as he thinks he has found, that there 

 is no reversibility, that means there is no transversion with EMS. 



Zamenhof: I would like to comment on the possibility that purines 

 methylated on the 7th nitrogen (60) are kept in the molecule and are 

 reproduced. I do not think this is possible. We tried to incorporate 

 7-methyl guanine into DNA of purine-requiring mutants and were 

 unsuccessful. The methylated compounds possibly could be made by 

 the cell. There may exist enzymes for such synthesis, but there are no 

 kinases to make triphosphates from 7-methyl nucleosides or from 7- 

 alkyl nucleosides, with an alkyl group as big as mustard. It is sure 

 that such an alkylated purine has to disappear. The final outcome 

 of the mutation cannot be the same as the first step. 



Freese: But I disagree here. You have only shown that the 7- 

 methylated guanine, or whatever you have taken, cannot be incor- 

 porated into DNA. You have not shown that the 7-methylated 

 guanine in DNA cannot duplicate. 



Zamenhof: Well, this is not shown by the experiment I mentioned, 

 but there are no known kinases which make such a triphosphate. 



Freese: In the cell? 



Lederberg: You're talking about two different things. Freese was 

 suggesting that the ethylated guanine in the original molecule in 

 which it is produced might persist in that form, or might necessarily 

 or ultimately always give rise to a mistake in some of the subsequent 

 copies of that original DNA. Therefore, every initial event will 



