156 MUTATION AND PLANT BREEDING 



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- MUTATION FIXATION 

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POST-IRRADIATION INCUBATION (MINUTES) 



Figure 3. — Relation of "mutation fixation" (MF), as indicated by CMP- 

 challenge, to synthesis of RNA, DNA, and protein in a UV-irradiated 

 culture of Escherichia coli strain WP2. Following irradiation the bac- 

 terial 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. 

 At the indicated intervals aliquots were taken and CMP (20 micro- 

 grams/ ml) added to them. They were then incubated for an additional 

 45 minutes at 37 °C to allow CMP-promoted MFD to take place and 

 plated on M + 2.5 per cent broth agar. At the time that samples were 

 taken for CMP treatment other samples were taken for RNA, DNA, 

 and protein determinations. 



revealed by incubation in CMP or amino acid-deficient media. With 

 5-HU treatment decline in mutation frequency does not start 

 immediately but only after some 20 minutes incubation in its pres- 

 ence. Moreover, 5-HU promoted decline appears inversely correlated 

 with MF and with RNA synthesis. This evidence, which suggests that 

 5-HU is exerting its effect through RNA synthesis, would account 

 for the lag in initiation of the decline in mutation frequency. One 

 possible explanation for the 5-HU promoted decline in mutation 

 frequency is that the analogue is incorporated into new RNA, thus 



