264 ERNST FREESE 



transition luutaiits that are not rcN'cisihIc hy I'll^S oi- low |)I1 may have 

 been caused by transversions, (lelclions, or inscitioiis. 



3. Both transition and transversion mutants which aic \ciy suscepti- 

 ble to reversion by agents preferentially attacking G or C also fic(|ueiitly 

 revert spontaneously. It seems likely, therefore, that (!-(' pairs mutate 

 spontaneously more freciuently than A-T pairs. 



It must be emphasized that these tentative conclusions arc only valid 

 for the particular phage system examined. There is no justification for 

 a general statement about the frequency or the specificity of spontaneous 

 mutations in all organisms. One could argue that G and C are chemically 

 more vulnerable than A or T; but even that is true only for most of the 

 directly acting chemical agents and wrong for certain ones (e.g., perox- 

 ides, hydrazine). Besides, we do not know whether in a particular 

 organism most mutations arc caused by mistakes during DNA duplica- 

 tion or by chemical changes of DNA. A typical example for the differ- 

 ences of similar organisms has been reported by Kirchner (1960) for 

 Salmonella; he found that for one strain (LT2) 26% of the spontaneous 

 mutants could be induced to revert by AP or BUdR while for another 

 strain (LT7) 95% of the spontaneous mutants could be induced to 

 revert by these base analogs. The second strain differs from the first 

 one by a gene which increases the frequency of spontaneous mutations. 

 The result indicates that the mutator gene produces a substance that 

 induces transitions. 



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