HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 



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POSTIRRADIATION INCUBATION (MINUTES) 



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Figure 7.— Effect of CMP in an X-irradiated culture of Escherichia coli 

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

 sion was diluted 1:4 into M medium plus casein hydrolysate (2 mg/ml) 

 and dl-tryptophan (0.2 mg/?nl). This culture was then divided into 

 tivo portions. To one portion chloramphenicol (20 micrograms /ml) 

 was added immediately and incubation at 37°C started. The other por- 

 tion ivas incubated for 10 minutes at 37°C before adding chloramphen- 

 icol (20 micrograms /ml) and then incubation was continued. At the 

 indicated incubation times samples were withdrawn from each culture 

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



plates have become susceptible to CMP. Within 10 minutes incuba- 

 tion potential mutants can be divided into two classes, viz., those 

 completely expressed and not sensitive to CMP, and those which are 

 not capable of being expressed and are sensitive to CMP. 



If we assume that X-ray has the capacity to mutate genetic mate- 

 rial at two different levels, viz., (a) in the parental DNA, presumably 

 by direct physical action, and (b) in the daughter DNA, by some 

 copy-error mechanism, these results can be readily accounted for. 



