MUTAGENESIS 89 



One is that, until yesterday, I thought that bromouracil was the 

 only base analogue that is incorporated, but now I have heard that 

 2-aminopurine is also incorporated. But certainly, one does not assume 

 that caffeine or azaserine is incorporated in bacterial DNA? Is it so 

 sure — and this is the first question I wanted to ask — is it really quite 

 sure that all the bromouracil goes into DNA? I read a note by Dunn 

 in which he states that some thymine goes into RNA (46) . Could this 

 not apply also to bromouracil, and if so, could not the amount in- 

 corporated be different in bacteria that have been infected with phage? 

 My first reason for doubting the incorporation hypothesis is, therefore, 

 that many excellent mutagens are not incorporated, and that if a 

 small amount of bromouracil remains outside the DNA, it might be 

 this and not the incorporated one that is mutagenic. 



Secondly, in Litman's experiments (45) , there was very little cor- 

 relation between the amount of bromouracil incorporated and the in- 

 duced mutation frequency. In fact, even when 100 per cent of the 

 thymine had been replaced by bromouracil, there still were rather few 

 mutations. I expect one could say that only a certain probability exists 

 of an incorporated molecule producing a mutation, but there should 

 still be a correlation between the amount incorporated and the pro- 

 portion of mutants. 



Freese: No, not necessarily. If the mutation arises by a mistake in 

 incorporation and not by a mistake in duplication, there need be no 

 correlation between the amount of incorporation and the extent of 

 mutation. 



Auerbach: A mistake? But if a hundred mistakes are made? 



Freese: I mean a mistake in incorporation (23) as one by which the 

 bromouracil is, by mistake, incorporated by pairing with guanine in- 

 stead of adenine; one could think of the possibility that very little 

 bromouracil gets incorporated and yet many mutations are made by 

 the incorporation of the bromouracil in the place of cytosine. 



Auerbach: Has it ever been tested? For instance. Dr. Zamenhof has 

 strains which differ in the extent to which they incorporate bromo- 

 uracil, but has it ever been tested whether they also differ in mutation 

 frequency (88) ? 



Zamenhof: We have not tested it quantitatively. 



Auerbach: Dr. Freese, in one of your experiments, for a very specific 

 question, you mixed thymine with bromouracil in order to see whether 

 this affected specificity (25), but did this alter the over-all mutation 

 frequency? 



Zamenhoj : We have not tested it. But Dr. Greer in our laboratory 



