142 MUTATIONS 



spot; in fact, there exist here mutants which have a reversion rate of 

 orders of magnitude higher than the total forward mutation rate, which 

 is a composite of at least 400 sites. 



The fact we are faced with here, then, is that there exist, among 

 reverse mutations, types of events which are excluded among the 

 spectrum of forward mutations. 



Lederberg: Are you making a point of that? 



Benzer: Yes, but give me more time. This immediately raises the 

 question of whether reverse mutants cannot themselves represent a 

 composite of several different possibilities. 



There are many precedents for this in many genetic systems, where 

 suppressor mutations, at a different site, produce a false revertant. 

 In the case of r II, Feynman has actually demonstrated suppressor 

 mutations within the r II region. If you take a mutation at a particular 

 point, you can have other mutations at any of several points which 

 will suppress the effect of the first one so that the first mutation rate 

 you measure in such a case will be a composite of several. 



Zamenhof: Did you ever observe mutation from nonhot to hot 

 spot? 



Benzer: Nonhot to hot? You mean a mutation, a change in a cold 

 spot so it is now hot? 



Zamenhof: Yes, and also vice versa. 



Benzer: I can give you the opposite. I can modify the structure to 

 get rid of hot spots. 



Lederberg: Another more obvious point I was going to ask is, if you 

 take the temperature at the spots you actually find, are they unlikely 

 to be found in an extremely unstable state? 



Benzer: The point I'm trying to make is that when we're mapping 

 mutation frequencies in a forward direction, we're not looking at a 

 random selection of the types of molecular events which can happen 

 in a piece of nonsense genetic material, but we're looking at a structure 

 which has evolved in such a way as to eliminate the hot things. 



What this suggests is that those hot spots which still remain are 

 simply a matter of evolution not having reached its culmination. This 

 might suggest that in Dr. Demerec's system, where he doesn't find 

 hot spots, the bacteria have evolved a little further than phage has, 

 and have gotten rid of them. 



Freese: I would like to mention in this connection that phage con- 

 tains, first of all, hydroxylmethyl cytosine instead of cytosine, and 

 second, the hydroxylmethyl cytosine is glucosylated, and not all of 

 the HMC bases are equally glucosylated. I have reason to suspect 



