MUTAGENESIS 137 



described the technique of these experiments. The difference between 

 viabihty plates and mutation plates may lead to overestimates or 

 underestimates of the living spores on the mutation plates, that is, 

 of the spores that were actually tested for mutations. Hollaender and 

 his collaborators (36) found that X-rayed bacteria survive better on a 

 medium containing amino acids than on a medium not containing 

 them. When amino acid revertants are scored, the viability plates con- 

 tain the required amino acid, the mutation plates do not. The degree 

 of recovery depends on the type of medium in which the bacteria had 

 been grown before treatment. It might well depend also on the type 

 of treatment. 



This finishes what I wanted to say about mutagen specificity. I have 

 shown by examples that observed mutagen specificity need not reflect 

 specificity of reaction between mutagen and gene, but may occur at 

 any one of the many steps that together form the process of muta- 

 genesis. If one is interested in the chemical reaction between mutagen 

 and DNA, all these other sources of specificity are sources of error to 

 be avoided. If one is interested in the process of mutagenesis as a whole 

 they are tools for analyzing its successive steps. 



This leaves us, then, with the specificities which really seem to occur 

 at the level of the DNA, and I think there we have the best example 

 in Dr. Benzer's work on phage. He is going to report on that. 



Benzer: My attention has been restricted to a very small portion 

 of the genetic structure of T4 phage, the r II region. By very simple 

 methods it is possible to examine the location of mutations within this 

 region, with a very high degree of resolving power, so that we can draw 

 a magnified map and tell if a mutation is at this point or at another 

 point only one nucleotide step or more away. To measure mutation 

 rates at various points is simply a matter of isolating a lot of r II 

 mutants and seeing where they occur. 



The first results I want to show are obtained with "chronic" 

 mutagenesis, that is to say, spontaneous mutations. The result is that 

 mutations are distributed over a large number of different sites and in 

 a nonrandom way. This is evidence for higher mutability at certain 

 points. If you get many mutations at one point, over and above the 

 number expected due to a random distribution, this point is called, for 

 want of a better name, a "hot spot." 



Figure 21 shows the spontaneous mutation rate at different points 



Figure 21. Topographic map of the r II region for spontaneous 

 mutations. Each square represents one occurrence observed at 

 the indicated site. 



