152 



MUTATION AND PLANT BREEDING 



EFFECT OF POSTIMADIATION INCUBATION ON REVERSION OF E. COU WP2 



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MIN. * AA. 



MIN. * AA. ♦ DNP. 



40 60 



TIME (MINUTES) 



Figure 1. — Comparison of "mutation stabilization" (MS), "mutation 

 frequency decline" (MFD), and the effect of dinitrophenol on UV -induced 

 reversion of tryptophan requirement in Escherichia coli strain WP2 

 The UV -irradiated cells were incubated at 37° C in the following media 

 for the indicated time periods and then plated on M + 2.5 per cent nutri- 

 ent broth agar: (1) Min + AA = M medium + casein hydroiysate (NBC, 

 vitamin free, enzymatic, 2 mg/mi) and dl-tryptophan, 0.2 mgjml; (2) 

 Min + AA + DNP = medium (1) plus dinitrophenol (5 X 10' s molar); 

 (3) Min + AA + Chi = medium (1) phis chloramphenicol (20 micro- 

 grams I ml); (4) Min — N — M medium with ammonium sulfate deleted. 

 The plates were incubated 3 days then scored for mutation and survival. 

 No significant cJianges in survival level were produced by the above 

 treatments. 



curve in Figure 1). DNP specifically uncouples oxidative phosphoryla- 

 tion and also prevents MFD when it is added to nitrogen-free medi- 



