MUTAGENESIS 149 



other. But the ends do not seem to correlate with hot spots for point 

 mutation. 



Auerbach: Are there preferred size classes of deletions, or do you 

 get all classes, more or less? 



Benzer: It is difficult to answer the question in those terms. There 

 are certain deletions which tend to recur. The small ones tend to 

 recur. Then there are big ones which tend to recur, and also all kinds 

 of sizes in between. 



Neel: If I understand Dr. Auerbach's question, it implies that if you 

 plot the lengths of the deletions, you observe a normal curve. 



Benzer: I'm sorry, but that has not yet been done. I hesitate to 

 predict just what it would look like. The first step has been to carve 

 up the map with big deletions in order to obtain all the sites. Now 

 that we have identified many sites, we can use them to determine 

 the termini of all the deletions much more accurately than could be 

 done before. 



Auerbach: I wonder if it shows anything about the way the phage 

 chromosome is coiled? You may have loops. 



Benzer: Perhaps so. Let us turn to the results on induced mutations. 

 Here we have hot spots too. A mutagen may increase the frequency 

 at particular sites by a factor of 10,000 or more, while leaving un- 

 affected some other sites. 



Some mutagens that have been examined in this way so far by 

 various people are: nitrous acid, ethylmethane sulfonate, 2-amino- 

 purine, 2,6-diaminopurine, 5-bromouracil, 5-bromodeoxycytidine, pro- 

 flavine, ultraviolet light, heat at low pH, and hydroxylamine (7,8,9,24). 



Auerbach: The ethylmethane sulfonate, from what Dr. Freese said 

 this morning, was applied in vitro. 



Benzer: Yes, some of the mutagens such as nitrous acid were ap- 

 plied in vitro and others in vivo. 



Lederberg: For the record, you might say what you mean by in 

 vitro and in vivo. 



Benzer: In vitro means that you take phage in a test tube without 

 the bacteria, treat it with the agent, and then plate it on bacteria 

 to see if you have any mutants. In vivo means that you infect the 

 bacteria with phage and then add the mutagen while the phage is 

 multiplying. 



The figure shows the distribution in the r II region of mutations in- 

 duced by some of these mutagens. The top line is for spontaneous ; on 

 successive lines are nitrous acid, ethylmethane sulfonate, 2-amino- 

 purine, 2,6-diaminopurine, 5-bromouracil, 5-bromodeoxycytidine, and 

 proflavine. 



