HAAS, ET AL.: MUTATION INDUCTION IN BACTERIA 



157 



60 



., MUTATION FREQUENCY 

 INCUBATION IN 5"HU 



/ RNA SYNTH. 

 IN 5-HU 



RNA SYNTH. 



IN 6-AU 



J I 



2.2 



20 30 40 50 60 



MINUTES POSTIRRADIATION INCUBATION 



Figure 4. — Comparison of the effects of 6-aza uracil (6AU) and 5— 

 hydroxy uridine (5HU) on macromolecular syntheses and the freqeuncy 

 of induced prototrophs in UV-irradiated Escherichia coli strain WP2. 

 Following irradiation the bacterial suspension was diluted 1:4 into M 

 medium plus casein hydrolysate (2 mg/ml) and dl— tryptophan (0.2 

 mg/ml). This was immediately divided into two portions and 6AU 

 (0.05 micrograms /ml) ivas added to one portion and 5HU (0.05 micro- 

 grams/ml) to the other. Both cultures were then incubated at 37°C, 

 and after the indicated incubation intervals samples were taken from 

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

 the same times samples were taken for RNA, DNA, and protein analyses. 



producing nonfunctional RNA and using up the radiation-produced 

 mutagenic precursors. 



MFD is also obtained in the presence of the analogue 6-aza 

 uracil (6-AU) (Figure 4), and is similar with this analogue to that 

 obtained with CMP. Like CMP, 6-AU blocks protein synthesis, but 

 unlike CMP, this is brought about through direct blockage of RNA 

 synthesis. Addition of uridine will reverse the blockage to RNA and 

 protein syntheses and also prevent 6-AU promoted MFD. 



