144 MUTATIONS 



Auerbach: I don't know which way, only I should like to know 

 whether it does modify it. It should be excluded, I think. 



Neel: Yes, I agree with Dr. Auerbach. If, in another strain, you 

 found that hot spots were differently situated, this would certainly 

 influence your interpretation that the hot spots were a way station. 



Benzer: All the experiments I did were in one bacterium. If I do 

 one experiment in each of the different bacteria, I am distributing all 

 my effort in the wrong direction. 



Freese: It could be that the DNA polymerase has a specificity to 

 certain base combinations. 



Zamenhoj: Perhaps I could mention a few words about the hot spot 

 in bacteria, where such experiments can be performed with transform- 

 ing principle; in fact, we are doing it now, but no results as yet. We 

 have an unstable gene in which we have produced a hot spot or in- 

 stability by a mutagenic agent. We are extracting transforming princi- 

 ple from it, and we are now trying to transform a stable strain, which 

 is a different strain, by means of this DNA. If we can, that means 

 that the instability was due to hot spots in the DNA itself. If we do 

 not succeed, then, of course, negative results do not mean much; how- 

 ever, if they are taken at face value, it would mean that it was the 

 environment of the spot or DNA which produced hot-spottedness and 

 not this particular spot of the DNA itself. 



Benzer: Of course, everything that happens is the product of the 

 gene and its environment. The bacterium is the environment of the 

 phage. The point is, in a given environment, the different sites in a 

 cistron have different mutational responses. But I would rather change 

 specific parameters, like adding base analogues, than change from 

 one bacterium to another. 



Zamenhoj : In bacteria, there could be two cases. One could produce 

 hot-spottedness, which would disappear if one removed DNA and by 

 transformation put it in a different environment. But it could be the 

 other case, that the hot-spottedness was not due to a hypothetical 

 intracellular mutagen; it simply was a hot spot in the DNA itself. 

 It is important to differentiate between these two cases. 



Benzer: I would like to come back to the question of whether these 

 very high reverse mutation rates are merely a composite of things 

 that may happen at many sites. We test this by the following experi- 

 ment, which is a good deal of work, but it may be worth it: 



We measure the forward mutation rate at a particular site, from 

 the number of recurrences. Now take one of these mutants and from it 

 obtain a revertant. The question is whether this revertant is genuine. 



