MUTAGENESIS 151 



eigenstates for the spontaneous mutations? This is about the number 

 of levels that you usually pick up? Or is it just either hot or cold? 



Benzer: There are eight mutagens (including spontaneous) that have 

 been studied here. The number of possible combinations is 2 to the 8th 

 power, or 256. 



Lederberg: It is either "yes" or "no" for a given agent? You don't 

 quantify the different levels? 



Benzer: That is what I would like to do, but the data are not always 

 sufficient. The point I'm trying to make is that there are many kinds 

 of different spots. This is seen in the forward mutation rates of the 

 various spots, in the reversion rates for the various mutants, and in 

 the different patterns of response of various sites to different muta- 

 genic agents. One is forced to think in terms of neighbor interaction 

 between groups of bases, and not simply what happens to a single 

 base pair independently of its neighbors. 



Auerbach: Are there any coincidences between patterns of mutagens 

 which are more frequent than expected from random coincidence? Do 

 some of these mutagens have more overlap than the others in their 

 patterns? 



Benzer: Yes, of course. Those are the numbers which I was just 

 giving you. 



Auerbach: It was so quick that I couldn't follow it. For instance, the 

 diaminopurine and the bromouracil are very similar. 



Benzer: Hardly. Diaminopurine has 15 significantly hot spots, of 

 which only 2 have also been found hot with bromouracil. 



Neel: How many other bromouracil hot spots were there? What is 

 the rest of it? 



Benzer: Two out of 18 bromouracil hot spots. 



Auerbach: If you look at the mutagen rather than the spots, are 

 there any mutagens whose similarity of action on these spots is 

 noticeably greater? 



Benzer: Yes, there are certain similarities. A striking one is the 

 similarity between ethylmethane sulfonate and nitrous acid which, 

 according to the chemical indications, should act by utterly different 

 mechanisms. 



Freese: But they both attack preferentially the CG pair. 



Benzer: Yes, that's right. 



Auerbach: According to Vielmetter, the action on the guanine is 

 just the one which does not produce mutations in TMV. 



Freese: The action on the C is five times higher than that on the 

 A. 



