V. MOLECULAR MECHANISM OF MUTATIONS 229 



Since for the DNA bases the strongest reacting base is cytosine and 

 since the mutagenicity of this reaction can be explained, it seems 

 probable that the major mutagenic effect of hydroxy lamine in DNA is 

 due to the alteration of C. Both the unstable compounds (II or III) or 

 the final compound IV miglit be responsible for base pair changes. In 

 order to be sure of the details the frequencies of the tautomeric states 

 of the molecules in Fig. 10 should be known. In any event, the hydroxyl- 

 amino group should be more electronegative than the amino group (be- 

 cause of the electronegative oxygen) and hence at least the molecule III 

 or IV should be more frequently than C in the tautomeric state in which 

 a hydrogen atom is on the 3-position nitrogen. This tautomeric form 

 cannot pair with G but it can make at least one (and probably two) 

 hydrogen bonds with A. Thus one should expect that the effect of 

 hydroxylamine on DNA induces predominantly the base pair transitions 

 (Freese et al., 1961b). 



The specific effect of HA also explains why the reaction of hydroxylamine 

 with DNA has such a small lethal effect compared to the strong 

 mutagenicity. 



The reaction of hydroxylamine with phage T4 is somewhat more 

 complex since at low salt concentration HA mainly kills, apparently by 

 some reaction with the phage coat, and the mutagenic effect becomes 

 predominant only at high salt concentrations (>lilf NaCl) (Freese 

 et al., 1961a). These authors have also shown that for short reaction 

 times most ?'-type mutants appear in forai of mottled plaques while for 

 longer treatment the fraction of pure r plaques increases; one would 

 expect this since for longer treatment the likelihood increases that a 

 second reaction occurs in the non-mutated chromosomal strand which 

 causes it to produce non-viable or also r-mutant phages. HA induces 

 point mutations in phage T4 (Freese et al., 1961a). 



5-Bromouracil and its deoxyriboside react very rapidly with hy- 

 droxylamine, lose their bromine and their UV absorption irreversibly. 

 It is probable that this involves a succession of reactions but this has 

 not yet been examined thoroughly. T4 phages containing 5-bromouracil 

 in place of thymine are rapidly inactivated by hydroxylamine without 

 any concomitant increase in the frequency of mutations. Similarly, 

 hydrazine shows a much weaker mutagenic effect than hydroxylamine. 

 It seems that the breakage of the pyrimidine ring is much more fre- 

 quently lethal than mutagenic. The mutation frequencies of treatment 



