V. MOLECULAR MECHANISM OF MUTATIONS 



239 



nism 1 can be excluded as the major cause of mutations. Green and 

 Krieg (1961) have furthermore shown that the hibile ethyl groups can- 

 not indirectly induce mutations inside the bacterium since a coinfected 

 but nonalkylated phage does not reveal any mutation induction. These 

 authors have also observed, by single burst experiments, that EMS- 

 treated phages seem to continue throwing out normal and mutant phages 

 even after they have replicated; this is in agreement with both mecha- 

 nisms 3 and 4. 



In case 3 the frequency of mutants per viable phage would be inde- 

 pendent of subsequent incubation without DNA duplication while it 

 might increase in case 4. Such an increase has been observed after incu- 

 bation of free phages (Bautz and Freese, 1960) and for bacteria (Strauss, 

 1962). Besides that, mechanism 3 is expected to induce base pairing 

 mistakes only during DNA duplication and thereby only transitions, 

 while mechanism 4 should give essentially the same results as treatment 

 by low pH : the removal of guanine could result in transitions as well as 

 non-transitions. Non-transitions have actually been found (E. B. Freese, 

 1961) (see Table III). Hence mutations by ethylation of DNA seem to 

 be mainly caused by the ethylation of G, which often is removed from 



TABLE m 

 Reversion Induction of Spontaneously Reverting Mutants 



From E. Freese (1961) 



