V. MOLECULAR MECHANISM OF MUTATIONS 257 



tants. For this investigation one can use mutagens, which are expected 

 to induce only base pair transitions, but those in both directions (see 

 Section III) ; such agents are AP, BUdR, and NA. AH transition mutants 

 should be inducible to revert while non-transition mutants should not 

 respond. The two base analogs are especially useful for these studies 

 because simple spot tests can be employed to screen a large number of 

 mutants; such spot tests have been described for bacteria (Szybalski, 

 1958) and phages (Freese, 1959b). The reversion inducibility by the 

 base analogs has been determined for many rll mutants and the results 

 are given in Table III (p. 239). As expected, most of the mutants induced 

 by the transition-inducing agents AP, BUdR, HA, and NA can be induced 

 to revert by the base analogs. Several of them have also been tested for 

 reversion inducibility by nitrous acid and the result was positive (E. B, 

 Freese and E. Freese, 1961). The non-inducible mutants correspond to 

 the background of spontaneous revertants in the phage stocks. 



In contrast most of the spontaneous and proflavin-induced mutants 

 do not respond in this way. It seems clear that most of these non- 

 base analog inducible mutants must have been caused by base pair 

 changes different from transitions. Also about one-third of the mutants 

 induced by low pH or EES are of this type. It is significant that for these 

 mutants neither back mutations nor suppressor mutations are base 

 analog inducible (except for a few mutants whose spontaneous reversion 

 rates are so high that induction cannot be observed). 



Two-thirds of the mutants, induced by low pH and EES, can be 

 induced to revert by base analogs; they may contain a transition, or 

 possibly some other base pair change if the base analogs should only 

 induce suppressor mutations for some of these mutants. But EES and 

 low pH certainl}^ can induce transitions; this is also shown by their 

 ability to induce the reversion of transition mutants (see Section V,4). 



3. Further Subdivision of Non-Transition Mutants 



The mutants of Table III that cannot be induced to revert by the 

 two base analogs nevertheless revert spontaneously and genetic mapping 

 shows that they are point mutations. The question is which of these 

 mutants contain a transversion, a deletion, or an insertion? 



To answer this question, at least in part, one can measure the rever- 

 sion induction by the two mutagens, low pH and EES. About 70% of the 

 rll mutants, induced by these agents, can be induced to revert by the 

 base analogs; therefore many of them seem to contain a transition. We 

 have proposed before (Section III,D) that the gap produced by the 

 depurination of DNA could result in both transitions and transversions, 

 or in deletions. Since low pH and EES can induce transitions they 



