176 MUTATION AND PLANT BREEDING 



agents combine with bacteriophage in a two stage process; the first 

 step is a true combination, while the second is a decay event lead- 

 ing to phage inactivation (10). Only certain of these alkylating 

 agents produce bacteriophage mutations in vitro (9). 



There is a post-treatment decay reaction in phage plaque- 

 forming ability and in the activity of transforming principle from 

 B. subtilis following treatment with either the mutagenic ethyl 

 methanesulfonate or the nonmutagenic (for virus) methyl methane- 

 sulfonate. Incubation of bacteriophage T2 following treatment with 

 ethyl methanesulfonate does not greatly alter the frequency of r 

 mutants recovered. It seems likely that there is more than one site 

 of reaction of these alkylating agents with genetic material and it is 

 still difficult to determine the chemical difference between the muta- 

 genic and nonmutagenic reactions. Certainly it is established that 

 the reaction of alkylating agents with cellular constituents is a 

 complex process. 



The hypothesis of mutation due to base substitution is logically 

 satisfying but there are cases in which the mechanism by which 

 this substitution occurs is not yet apparent. Thymine starvation 

 is mutagenic and Kanazir (3) has shown that this mutagenic action 

 occurs during the first 30 minutes of thymine starvation and before 

 there is any lethality due to the thymine starvation. DNA duplica- 

 tion does not occur. To my knowledge there is no way at present 

 to account for this mutagenicity. Caffeine and other methylated 

 purines are mutagenic (11), but these compounds are not incorpo- 

 rated or metabolized (5) and presumably act by inhibiting some 

 enzyme involved in DNA metabolism. We do not know how this 

 inhibition results in a change in base order. 



Breaking a single one of the chains of the DNA double helix 

 can be mutagenic. Radioactive decay of P-32 leads to a transmu- 

 tation event in which the phosphorus changes to S-32. In most of 

 the decay events the daughter nucleus recoils from the molecule 

 leaving a "hole". Kaudewitz, et al. (4) have shown that this decay 

 event is mutagenic and that the production of mutation is inde- 

 pendent of the (3 radiation accompanying the radioactive decay. 

 Kaudewitz reports (personal communication) that about 30 to 40 

 per cent of the mutants obtained were able to revert to the wild- 

 type, indicating that a proportion (at least) of the strains obtained 

 were true mutants as distinguished from deletions. The removal 



