150 MUTATION AND PLANT BREEDING 



prior to irradiating increases the induced mutation frequency some- 

 what, although the effect is not as marked as with UV. 



II. Effect of Postirradiation Treatments on the Fate of Potential 



UV-induced Mutations 



Witkin (26) has suggested that the immediate postirradiation 

 synthesis of protein is necessary for expression of induced prototrophs 

 with certain auxotrophic strains of E. coli and Salmonella typhimuri- 

 um. In her experiments, expression of induced prototrophic mutants 

 was directly related to availability of a complex supply of amino acids 

 during the first hour of postirradiation growth. Chloramphenicol 

 (CMP), which specifically interferes with protein synthesis, prevented 

 mutation expression if the cells were treated during the first post- 

 irradiation hour. We have confirmed the existence of similar relation- 

 ships for the color mutations of E. coli strain B, and have shown that 

 the mutation frequency increase due to preirradiation incubation in 

 purines and pyrimidines is also dependent on postirradiation amino 

 acid supply. Involvement of postirradiation macromolecular syn- 

 theses (protein, RNA, DNA) in the mutagenic process has been 

 studied for both color mutants of strain B and for prototrophic muta- 

 tions of various auxotrophic strains. While the results were essentially 

 comparable, for technical reasons most of these postirradiation studies 

 were accomplished with the latter system. 



Mutation frequency decline 



Involvement of metabolic processes in mutation induction has 

 been studied by observing the effects of various conditions limiting 

 to amino acid or protein synthesis on UV-induced mutation (3, 4, 8). 

 Immediately following irradiation the cells were incubated in 

 nitrogen-free medium for periods of time varying from to 90 

 minutes, prior to plating on M plus 2.5 per cent nutrient broth agar. 

 Using this technique, it was found that when nitrogen-dependent 

 synthetic activities are limited there is a rapid decline in induced 

 mutation frequency obtained at plating on nutrient broth-containing 

 medium. The decline in mutation frequency is observed almost 

 immediately and proceeds to a level unaffected by further incubation 

 within 20 to 30 minutes at 37° C. The decline rate is similar in cul- 

 tures incubated in M medium and in purine-pyrimidine-supple- 

 mented medium prior to irradiation. Evidence that the processes 



