116 



BIOCHEMISTRY: BENZER AND F REESE 



Proc. N. a. S. 



SPONTANEOUS a 



INDUCED BY B a 



BROMOURACIL y 



••gmtnl (T) 



(D (D ® 



S PONTANEOUS 



PI w H H_B H SI n ca 



INDUCED BY 1 



BROMOURACIL \ / \__ 



J \ / \ / V 



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CISTRON 8 



B 



c 



Fig. 2. — Genetic maps showing the locations of spontaneous and 5-bromouracil-induced 

 mutational alterations in the rll region of phage T4 divided into cistron A and B. Each mutant 

 is represented by a box, placed in the proper segment of the map and shaded to indicate its re- 

 version index. The relative order (but not the lengths) of segments 1, 2, 3, 4, 5, 6, 8, and 10 have 

 been established. Segments 7, 9, and 11, are lumped together and considered as one segment. 

 The order and position of different mutations within one segment have not been determined. 



they were leaky. The results of mapping the remaining 67 mutants are shown in 

 Figure 2. This set of mutants reveals "5-bromouracil hot spots" that are located at 

 different positions from the spontaneous ones. 



It must be emphasized that the vertical scales in Figure 2 are very different for 

 the two sets of mutants. The over-all induced mutation rate was several hundred- 



224 



