MUTAGENESIS 141 



20 times higher than the probability of mutation per replication. This 

 number for r // mutants, or for both cistrons together, is approximately 

 2 X 10"^. It is the custom to call this the "mutation index." (Divided 

 by 20, that would correspond to a probability of 10"^ per replication of 

 getting an r II mutation, added up over all these sites.) 



The total forward mutation index, which is a composite of many 

 sites, is 2 X 10"^. This means that the hottest spot in the map, at 

 which 500 recurrences were observed out of about 2000, accounting 

 for one fourth of the total mutation rate, has a mutation index of 0.5 

 X 10-^. 



Freese: Excuse me, but is that per replication? 



Benzer: That is mutation index as defined here. What is actually 



measured is — . I am using the same measure throughout, so that the 



N 



comparison, in talking about forward and reverse mutations, will be 

 valid. The conversion factor to frequency per replication will be con- 

 stant throughout. 



The range of forward rates observable is limited since the coldest 

 spot observed is determined by how many mutants you have mapped. 

 The coldest spots here would have zero occurrences out of 2000, or a 

 mutation index of less than 1 X lO"'*'. 



If we take each mutant and measure its reversion rate, we can ob- 

 serve a much larger range because we can select for reversions. To 

 measure reversion rate, you do exactly the same experiment, except 

 that you now start with a few mutant particles, so the probability 

 of adding revertants to the tube is negligible. Let them grow up 

 (using a bacterium, where there is no selection between the mutant 

 and the standard type) so they develop into the same size population as 

 before, and you now measure the fraction of revertants. If you do this 

 with various r II mutants, you find mutation index values which go 

 from less than 10"^ up to as high as 10"^. 



So far as I can see, there are about as many grades in between as 

 one can determine experimentally. It is difficult to distinguish a factor 

 smaller than 2 because of statistical fluctuations. 



Lederberg: What is the smallest possible number? 



Benzer: The smallest nonzero number is 10'^. This is limited by the 

 sensitivity of the test. 



Lederberg: There is no obvious gap? 



Benzer: That's what I mean to say. I haven't seen any obvious 

 gap. There are many different rates. The other point is that there exist 

 reverse mutation rates which are higher than for the hottest forward 



