MUTAGENESIS 133 



because each mutant presumably — and I guess we'll hear from Dr. 

 Benzer about this — will have its own particular pattern of back 

 mutation, depending on the site where it is mutated. 



Auerhach: There is another example from Witkin's work which may 

 be relevant here. In her first paper, when she reported on the stabiliza- 

 tion process, she also had tested leucine reversion and found it did not 

 require protein in the plating medium. Leucine reversions and trypto- 

 phan reversions are both reverse mutations from auxotrophy to proto- 

 trophy, and if you had a leucineless strain and plated on minimal 

 medium, UV would seem to act in a specific way on the leucine re- 

 versions and not the tryptophan reversions. 



Goldstein: All I'm saying is that in going from auxotrophy to proto- 

 trophy you don't really know whether you're going backward or for- 

 ward. One should use a system that is unequivocally forward for 

 studying this type of comparison between genes. 



Auerhach: What I said last was that the stabilization process might 

 be influenced by the mutagenic treatment used. This seems to have 

 been so in an experiment on tryptophan reversions in E. coli by Strauss 

 (70). He treated with UV or with diepoxybutane. After treatment, he 

 postincubated for one hour in liquid medium with or without a trypto- 

 phan analogue which prevented protein synthesis, and after this he 

 plated on broth. Under these circumstances UV induced reversions 

 were inhibited, while chemically induced ones were not. Strauss in- 

 terprets this to mean that no protein synthesis is required for stabiliza- 

 tion after treatment with diepoxybutane, but to me it seems equally 

 likely that after the slowly acting chemical treatment one hour post- 

 incubation is not enough for the effect to show up, for when plating 

 was done on minimal medium instead of on broth, chemically induced 

 reversions, too, were inhibited by incubation with the tryptophan 

 analogue. Whatever the interpretation, operationally this is again a 

 case of apparent mutagen specificity, in which the seat of the speci- 

 ficity is not the reaction between mutagen and gene but a later step 

 in mutagenesis, probably stabilization of a premutated state. 



Once a mutation has been produced, the mutated gene has to 

 establish itself and start a new biochemical pathway in the cell. 

 After adenine reversions, the cell has to start making adenine, and 

 after inositol reversions it has to start making inositol. To me it seems 

 entirely conceivable and even very probable that the same chemical 

 treatment which produces the mutations may interfere with the bio- 

 chemical resources and potentialities of the cell in such a way that 



