250 ERNST FREESE 



have l)cen oxainined in sufficient detail, rcvcrtants have been found 

 wliich l)y functional and ivcoinhinational tests are indistinguishable from 

 the standard type phages (Freese, 19')9b; P'reese ct al., 1961b). When a 

 new sjiectruni of rll forward nuitations was deterniined a few revertants 

 behaved as standard tyi)e phages (Benzer, 1961). It is veiy likely, 

 therefore, although inipossil)le to prove unequivocally by jiurely genetic 

 means, that genuine back mutations to the standard genotyi)e exist for 

 most /•// mutations and hence that the original cliangc in DNA was 

 rather small. 



Exceptions to this rule may he the nuitants induced by jiroflavin 

 (Crick et nl., 1961) which spontaneously seem to revert only, or mostly, 

 by suppressor mutations. Otherwise the existence of suppressor mutations 

 is unimportant for the abo^'e argument. 



C. THE KINETICS OF MUTATION INDUCTION 



The ease with which most chemical reactions can be both initiated 

 and stopped enables one to study the increase of the mutations frequency 

 as a function of the number of nucleic bases attacked. Phage T4 is 

 usually inactivated by a first-order reaction kinetics: 





= g-^' = e-» 



Here B is the titer of the T4 phages on bacteria B, B,, is the titer at 

 zero time of treatment, /? is the inactivation constant and n is the number 

 of "lethal hits," n is proportional to the number of deaminated bases 

 (as long as that number is small) ; it is not identical to the number of 

 deaminated bases since there should be many bases which do not have 

 any important informational task and thus can be altered without lethal 

 elTect. 



(3 depends on the concentration of the mutagen. If that is constant in 

 time, as for the treatment by nitrous acid, the time of treatment itself is 

 proportional to the number of bases attacked. When, however, the 

 mutagen decays rather rapidly,'* e.g., by hydrolysis as for many alkylating 

 agents, the more adequate measure for the number of bases attacked is: 



1 Bo 

 n = In -5- 



D 



* Once the single-hit character of mutation.s of a certain kind (e.g.. forward muta- 

 tions of a certain phenot.ype) is established one can use their frequency as another 

 measure for the number of bases attacked. One then plots against this frequency 

 the frequency of another kind of mutation. Such a plot has the advantage of being 

 independent of the concentration of the mutagen or of an assumed constant corre- 

 lation between lethal hits and bases attacked, which sometimes does not hold true 

 (Freese et al, 1961a). 



