138 MUTATIONS 



of the structure. It represents the distribution of approximately 2000 

 spontaneous mutations. 



As far as the sequence of sites is concerned, this is known only 

 segment by segment, that is, the mutations of one segment are ordered 

 with respect to those of another segment, but within a segment the 

 order is not yet established. Some sites which are empty in this figure 

 are known from other mutations, but the occurrences shown are 

 restricted to spontaneous ones. 



What is striking is the two very prominent hot spots; at one site, 

 500 recurrences have been observed. There is only one site in the 

 entire structure that has that degree of mutability. There is another 

 one that has about half the mutation rate, and there are spots of 

 decreasing "temperature," coming down to sites at which only a single 

 mutation has been observed, and then other sites which we know must 

 very well exist but have not yet been observed. 



Lederberg: How many sites are there? 



Benzer: This map contains 308 known sites in the two cistrons of 

 the r II region. 



Lederberg : How many nucleotides? 



Demerec: Where is the break between A and B? 



Benzer: The number of nucleotides is not known, but it can hardly 

 be more than a few thousand by the most conservative estimate. The 

 break between the A and B cistrons is shown in the figure. From the 

 beginning to the broken line is the A cistron, and the rest is the B 

 cistron, which is about half as long. The B cistron also has about half 

 the total mutation rate, but this correlation with size is almost mean- 

 ingless by virtue of the fact that half the total mutation rate of the 

 whole B cistron resides in one single spot. 



From the statistics of this distribution one can estimate how many 

 more sites there must be at which spontaneous mutation has not yet 

 been observed. This can be done from the coolest spots, taking the 

 sites of one occurrence, and the sites of two occurences, on the as- 

 sumption that these represent the lowest mutable class of sites, and 

 you can calculate the number of zeros there must be. This comes out 

 to be another 120 or so. This is, of course, a minimum estimate be- 

 cause many of the cools may well be slightly warm spots. In any case, 

 the total mutation rate for r II mutations must be a composite of at 

 least 400 sites. 



The mutabilities of the various sites cover a range (at least) of 

 500 to 1. So far as the distribution of these sites throughout the 

 structure is concerned, there is no evidence of any large quiet regions 

 where mutations are rare. 



