HAAS, ET AI..: MUTATION INDUCTION IN BACTERIA 



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PLATING MEDIUM- 

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PLATING MEDIUM- 

 "M" AGAR 



CHLORAMPHENICOL CHALLENGE 



10 20 30 40 50 60 70 



POSTIRRADIATION INCUBATION (MINUTES) 



80 



Figure 8. — Separation of X-ray -induced mutations in Escherichia coli 

 strain WP2 into two classes by CMP challenge followed by plating or 

 M agar and broth-supplemented M agar. Following X-ray irradiation 

 (10 kr), the bacterial suspension was diluted 1:4 into M medium plus case- 

 in hydrolysate (2 mg/ml) and dl— tryptophan (0.2 mg/ml) and incubated 

 on a reciprocal shaker at 37° C. At the indicated times CMP (20 micro- 

 grams/ml) was added to aliquots of the culture and incubation continued 

 in the presence of CMP for 40 additional minutes. All samples were then 

 plated on M agar and on M plus 2.5 per cent nutrient broth agar. 



half of the tyro 1 mutations were able to be expressed on this medium 

 after 20 minutes incubation. During the 20 to 40 minute incubation 

 period no more mutations were expressed. At this time thymine was 

 added to the incubation tubes and the incubation continued, and 

 during the next 20 minutes the remaining half of the mutations were 

 expressed. Therefore, after 60 minutes the mutation frequency 

 obtained was the same as that of the control, which had been 

 incubated in complete medium. No DNA synthesis was observed in 

 the test culture until thymine addition, and then DNA synthesis 



