MUTAGENESIS 97 



meric shifts is really not known. If we take the frequency with which 

 bromouracil induces mutations we can estimate that the ratio of bromo- 

 uracil, being in the normal state to the one being in the tautomeric 

 form, might be of the order of, let's say, 10"'^. In contrast, the 2- 

 aminopurine-cytosine base pair might have the right distance and 

 angle in perhaps 10"^ of the cases in which such a pair is formed. 



In the case of adenine (Fig. 18) you have the amino group in the 6- 

 position. You have to look at the molecular model. The two amino 

 groups interfere; there is no hydrogen bond possible between two 

 amino groups, and there are no hydrogen bonds possible otherwise. 



Benzer: What do you expect with 2,-6-diaminopurine? 



Freese: I would say that the same argument applies as for 2- 

 aminopurine; 2,-6-diaminopurine can also pair with cytosine by a 

 hydrogen bond at the 2-position. 



Benzer: No, it can't pair in the 6-position. 



Freese: The 6-position should interfere, and therefore the effect of 

 2,-6-diaminopurine should be less than that of 2-aminopurine. But it 

 should be mutagenic. 



Benzer: It shouldn't work at all. It is impossible to form this base 

 pair. 



Freese: No, I mean this base pair can be formed. 



Benzer: You can't form the complementary pair of bases. You have 

 two amino groups knocking head on. 



Freese: But I'm not making any other base pair at the 6-position. 



Benzer: You won't be able to make a proper base pair with two 

 amino groups. 



Freese: Why not? I can move the molecules sideways. The question 

 is only whether it gets incorporated. I certainly can make that one 

 hydrogen bond in the 2-position. 



Auerbach: I wanted to say something similar. In Vielmetter's 

 scheme, hypoxanthine is an intermediate. That, I think, is incorporated. 

 But does it produce mutations? 



Freese: No, hypoxanthine does not get incorporated into DNA. 



Auerbach: I thought I saw in one of Romberg's papers that it was 

 incorporated. 



Freese: Oh, yes, in Romberg's papers it is incorporated. That is 

 quite correct. He has to make artificially the hypoxanthine deoxynu- 

 cleoside triphosphate. 



Auerbach: How do you make that? Does he feed it to a phage? 



Freese: No, you can't get any nucleotides into a bacterium. This 



