MUTAGENESIS 143 



that some of the hot-spot effects in these phages are due to differences 

 in isolation. 



Benzer: What is your reason for this? 



Freese: Hydroxy lamine reacts with hydroxy Imethyl cytosine more 

 easily than with cytosine. It does not react with methyl cytosine. It 

 does not react with pseudo-uridine but it does react with uridine. In 

 other words, whenever one puts on the 5-position of cytosine a methyl 

 group or a sugar, it reacts less; the reaction goes down. I have not 

 yet measured — because I have not had the compound — the reaction 

 with the glucosylated HMC and the nonglucosylated HMC, but be- 

 cause we have certain mutagenic results which would indicate that 

 there is a difference, I suspect that this possibility should be looked 

 into. 



Lederberg: There is a way to test this. 



Benzer: This specific point? 



Lederberg: Yes, I think so, that is, to compare the mutation spec- 

 trum of T2 with T2 that has once been crossed with T6. Has that 

 ever been done? 



Benzer: No. I am in the process of comparing the spectrum of T4 

 with the spectrum of T6, to see whether they differ. As far as it has 

 gone, I can say that the two major hot spots that T4 has are still 

 present in T6, but appear to have different relative mutation rates. 



Lederberg: It might be more appropriate to use an Fi hybrid or 

 back-cross stock which has the same DNA as T-4. 



Benzer: Yes, I know. The difficulty with that is that you have no 

 guaranty that the glucosylation really applies in detail within the r II 

 region. 



Auerbach: Is it possible to test the influence of the genotype of the 

 bacterium on this spectrum? 



Benzer: My only answer to that is that I have enough troubles. 



Auerbach: But on this depends the interpretation of the hot spots, 

 doesn't it? 



Benzer: I don't see why, because the whole beauty of the system, if 

 it has any beauty, is that it is restricted to one type of mutant, one 

 particular region of the phage, and everything else is kept constant 

 except the thing that you're looking at. What I'm interested in is the 

 internal specificity of the structure. 



Auerbach: Yes, that may be too troublesome, but in bacteria, 

 mutagen specificity is under the influence of the residual genotype. In 

 your case, it might be under the influence of the bacterial host strain. 



Benzer: In which way could you expect that to modify the result? 



