HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 161 



thymine plus methionine medium. However, when thymine is 

 restored to the incubating culture, DNA synthesis resumes and 

 mutation expression is observed on the latter plating medium. 



While processes leading to mutation induction occur prior to 

 DNA synthesis and involve macromolecules other than the DNA, the 

 mutation apparently comes finally to reside in new DNA but is not 

 expressed until this DNA has functioned to establish the requisite 

 enzyme. 



III. Involvement of Postirradiation Macromolecular Syntheses 

 in X-ray-Induced Mutation 



Considerable time has been devoted recently to examining the 

 relation of postirradiation macromolecular synthesis to X-ray-induced 

 mutation. Earlier it was stated that preirradiation supplementation 

 experiments indicated that a part of X-ray-induced mutations are 

 due to incorporation of radiation-altered precursors during post- 

 irradiation synthetic processes (7, 8). Because of this we have been 

 examined the postirradiation synthetic events of X-irradiated bacteria 

 for the same processes involved in UV-induced mutation. The tech- 

 niques and methods elaborated for the UV work were used in the 

 X-ray studies, except that the bacteria were at a higher concentration 

 during exposure. The total dose in most experiments was 10 Kr given 

 at a rate of 2860 r per minute. The cells were held in an ice bath 

 during exposure. 



Figure 6 presents results obtained when X-irradiated cells are 

 plated on M agar and on broth-supplemented M agar after various 

 intervals of postirradiation incubation in complete medium. It is evi- 

 dent from the M agar plates that an appreciable portion of the total 

 mutational yield is expressed after only 10 minutes incubation in 

 complete medium. With UV irradiation a much lower proportion of 

 prototrophs were expressed prior to initiation of DNA synthesis (see 

 Figure 5). In the case of X-ray exposed cells, during the first 10 

 minutes of postirradiation incubation approximately 40 per cent of 

 the prototrophs are expressed in the absence of measurable DNA 

 synthesis. DNA synthesis is initiated after 10 minutes and the M agar 

 platings indicate that additional mutations are expressed following 

 initiation of DNA synthesis. 



When protein synthesis is inhibited by adding CMP immediately 

 after irradiation, there is no appreciable effect on mutation frequency 



