200 ERNST FREESE 



should also be able to induce transvcrsioiis. W'iiether they induce dele- 

 tions is unknown. 



The non-transition nuitants were therefore tested for their reversion 

 inducibility by EES or low pH. The results of such tests are summarized 

 in Table IV (E. B. Freese, 1961). The values a/p in this table are the 

 slopes of the straight line shown in Fig. 17. Each slope has been deter- 

 mined by several points. Several spontaneous and EES induced mutants 

 show a small increase in the frequency of revertants per viable phage, 

 with both low pH and EES, while no such increase was observed with 

 nitrous acid. Several induced revertants of the two EES-induced nm- 

 tants, EES 66 and EES 116, were examined further and behaved in 

 functional tests as well as in backcrosses as if they arose by back muta- 

 tions (E. B. Freese, 1962). It thus seems likely that the non-transition 

 mutants which are inducible to revert by low pH and EES have a trans- 

 version at their mutant site. 



The strongly responding nuitants of Table IV probably carry a G-C 

 pair at their mutant site, because EES and low pH seem to remove G 

 much more frequently than A under the conditions used for the induc- 

 tion of mutations. The G-C pair could stem either from a T-A or a 

 C-G pair in the standard DNA. But whether the less strongly respond- 

 ing mutants also have a G-C pair and just react less because of some 

 higher mutagenic specificity or whether they have an A-T pair is not 

 clear. 



Some spontaneous and most of the proflavin-induced mutants cannot 

 be induced to revert by EES or low pH. This is complemented by the 

 observation of Orgel and Brenner (1961) that most proflavin- or 5- 

 aminoacridine-induced and some spontaneous mutants can be induced 

 to revert by proflavin or 5-aminoacridine, while BU induced mutants 

 cannot be induced by these agents to revert. If the hypothesis of Brenner 

 et al. (1961) is correct, mutants inducible by acridine dyes should con- 

 tain a deletion or insertion of one nucleotide pair. So far the experi- 

 mental results do not permit a decision between this possibility and the 

 alternative that these mutants contain a transversion with an A-T pair. 



4. The Preferred Midogenie Direction of Transition Inducers 



Some transition-inducing mutagens react preferentially or predom- 

 inantly with one of the DNA bases. It should be possible to utilize 

 these agents in order to subdivide transition mutants further according 

 to A-T and G-C pairs at the mutant site. Thus nearly all forward 

 mutants induced by HA and most of the transition mutants induced by 



G A 

 EES or low i)H should contain a base pair change -^ . 



C T 



