HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 155 



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"MUTATION STABILIZATION 



MUTATION 

 FIXATION" 



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PROTEIN 

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POST-IRRADIATION INCUBATION (MINUTES) 



Figure 2. — Comparison of "imitation stabilization" (MS) and "mutation 

 fixation" (MF) in Escherichia coli strain WP2. Mutation followed was 

 reversion of the tryptophan requirement. Following UV irradiation 

 cells were incubated at 37°C in M medium plus casein hydrolysate 

 (2 mg/ml) and dl— tryptophan (0.2 mg/ml). Culture aliquots ivere plated 

 at the indicated times (MS) on broth-supplemented M agar. Also, after 

 these intervals of incubation chloramphenicol (CMP) (20 micrograms /ml) 

 was added to samples which were incubated for an additional 45 min- 

 utes before plating. This latter procedure is termed "CMP cliallenge" 

 and measures MF since all mutations remaining subject to the CMP- 

 promoted MFD process at the time of CMP addition are eliminated 

 during the subsequent 45 minutes incubation. Relative protein content 

 of the culture at the times of CMP addition is also given. 



about the same time and MF attains its maximum when the RNA 

 has doubled in amount. It thus appears that DNA synthesis per se 

 may be segregated from the MF process. 



The uridine analogue, 5-hydroxyuridine (5-HU), when present 

 during postirradiation incubation, promotes marked decline in muta- 

 tion frequency (Figure 4). However, this is not the MFD process 



