162 



MUTATION AND PLANT BREEDING 



120 - 



uj 60 



Q. 



CO 



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 Q. 

 O 



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Q. 



30 



PLATING MEDIUM- 



plating medium 

 "m"-agar 



DNA SYNTHESIS 



2.0 



1.75 



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 o 



1.50 : 



3 

 o 



> 



i- 

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.25 



20 30 40 



P0STIRRADIATI0N INCUBATION (MINUTES) 



50 



Figure 6. — Comparison of mutation frequency expressed on M agar with 

 that expressed on broth-supplemented M agar, and the relation of DNA 

 syjithesis to mutation expression in an X-irradiated culture of Escher- 

 ichia coli strain WP2. Following X-ray irradiation (10 kr) the bacterial 

 suspension ivas diluted 1:4 into M medium plus casein hydrolysate 

 (2 mg/ml) and dl— tryptophan (0.2 mg/ml) and incubated at 37° C. At 

 the indicated times aliquots were withdrawn and plated on M agar 

 and on M plus 2.5 per cent nutrient broth agar. Identical samples were 

 also analyzed for DNA content at each time. 



observed with plating the cells on broth-supplemented M agar after 

 various intervals of incubation. However, when plated on unsupple- 

 mented M agar, the mutation frequency is quite low (Figure 7). While 

 a large fraction of the potential mutations are not susceptible to CMP 

 (are not unstable and subject to MFD as in UV irradiation), it is 

 obvious that their expression is prevented by CMP. Therefore, all 

 X-ray-induced mutations, as wll as UV-induced mutations, evidently 

 require a period of protein synthesis following establishment in 

 the genome if they are to attain expression. When CMP is added after 

 10 minutes incubation in complete medium (Figure 7) it is found 

 that a large part of the mutations appearing on broth-supplemented 



