156 MUTATIONS 



Benzer: I'm not sure. It is a different kind of mutant, the host range 

 mutation in phage. 



Goodgal: From five different levels out of 50, or four different levels? 



Benzer: I seem to recall that some of them were obtained by a kind 

 of reversion at the site, so that they are not all obtained as forwards. I 

 think he had forward mutations, and then several revertants with 

 different degrees of return to the original type. How many of these 

 could possibly be suppressor mutations, it is a little difficult to con- 

 clude definitively. 



Atwood: Just one more question. Is the small number of reversion 

 classes the principal reason you have for thinking that the hot spot 

 is not a composite of more than one position? 



Benzer: Not at all. Position is based entirely on the absence of 

 recombination. 



Atwood: Do you have any other reason besides the small number of 

 reversion classes for thinking that there is not a composite of several 

 base pairs unable to recombine within a hot spot? 



Benzer: The reasons for believing two mutations to be at the same 

 site is the failure to detect recombination between them, even in a 

 frequency that would be expected if they were only one nucleotide 

 apart. 



Atwood: One can just as well say that there are variations along the 

 chain, in frequency of recombination, that account for this, then. 



Benzer: Yes. One could imagine regions where recombination is sup- 

 pressed. Anything I show as a hot spot is always subject to the possi- 

 bility of being split up by a more sensitive test, and if I indicate two 

 mutants at the same point, it only means that I have not succeeded so 

 far in demonstrating, by recombination, that they are different. The 

 only thing you can demonstrate in recombination experiments is that 

 two mutations are at different points. You cannot prove they are the 

 same. 



Lederberg: Would you dispute the former hypothesis that the 

 individual units of mutation are nucleotides, and that there are 

 definite spacing factors, so recombination takes place between groups 

 of three nucleotides? That would give you the number of varieties of 

 configurations you want for the different states that you described. It 

 might make some physiological sense, too. 



Benzer: I don't think I can really prove that that is not the case, 

 although I don't believe it for a moment. 



Lederberg: 1 don't either, but I would just like to know how we dis- 

 pose of it. 



Benzer: If one had a good estimate of the correlation between re- 



