160 



MUTATION AND PLANT BREEDING 



T 



10 20 30 40 50 60 70 



POSTIRRADIATION INCUBATION (minutes) 



Figure 5. — Relation of "mutation fixation" (MF) and "mutation expres- 

 sion" to macromolecular synthesis in a U V -irradiated culture of Escheri- 

 chia coli strain WP2. Folloiving irradiation the bacterial suspension was 

 diluted 1:4 into M medium plus casein hydrolysate (2 mg/ml) and dl- 

 tryptophan (0.2 mg/ml) and incubated at 37° C. MF was determined 

 by the "CMP -challenge" method (see Figure 2) followed by plating on 

 M phis 2.5 per cent nutrient broth agar. "Mutation expression" xuas 

 determined by plating directly onto M agar after the indicated periods 

 of incubation in M medium plus casein hyrdolysate and tryptophan. 

 Prototrophic mutations are expressed as the percentage of maximum 

 mutation frequency response (80 phototrophs/10 6 UV survivors when 

 plating was on broth-supplemented M agar, and 60 prototrophs/10 6 UV 

 survivors when plating xoas on M agar). At the same times that samples 

 ivere taken for CMP challenge and for plating on M agar, samples were 

 also analyzed for RNA, DNA, and protein. 



irradiation incubation, no DNA synthesis takes place and mutation 

 expression does not occur with subsequent plating on M plus 



