152 MUTATIONS 



Lederberg: Given this much wider range of discrimination, could you 

 now say there are not more than four states at any one site? 



Benzer: Yes. Ernst tested this one which I mentioned before. It is a 

 hot spot, but has three different kinds of mutation distinguished by 

 their reversion rates. He tested these for induction of reversions by 

 bromouracil and 2-aminopurine. 



Lederberg: Out of how many incidents? 



Benzer: Out of how many occurrences at that site? Several 

 hundred. 



Lederberg: But was it tested with respect to number of different 

 agents? You see, it is much more versatile. 



Freese: I found that one of them can be induced to revert by 

 bromouracil and aminopurine, while the other two cannot. 



Lederberg: But there are several recurrences within each of these 

 states. 



Novick: His question was, can these states be split? I think that is 

 his question. 



Lederberg: I am looking for splitting of the state by looking at 

 several recurrences within them. If that has been done — 



Benzer: That is what we are talking about. Reversions can also be 

 split. There are many cases where a mutant reverts to two different 

 classes, one of which looks like the real standard type and the other 

 looks like a partial revertant. 



Lederberg: No, that is certainly applicable, but I think that the 

 data could be put to a very sensitive test with the idea that there are 

 really only four states. To take one of your hot spots in forward 

 mutation, this gives you four classes with respect to spontaneous 

 reversions — or, excuse me, three classes — which are presumably the 

 three alternative bases having been substituted at that point. If that 

 is true, these individual states which include several representatives 

 each, several incidents of mutation each, should not be further split 

 by using this large a battery of tests, even though a wider variety of 

 mutants would be split. 



Benzer: You mean there shouldn't be any new classes coming up, by 

 using chemical mutagens? 



Lederberg: Yes. 



Benzer: Actually, the mapping has only now reached the point of 

 getting everything in its proper site, so the examination of all the 

 classes has not yet been done. It should prove very interesting. 



Lederberg: Of the four classes of reversions, are there four classes of 



