Vol. 44, 1958 BIOCHEMISTRY: BENZER AND FREESE 119 



SUMMARY 



A set of spontaneously arising rll mutants of phage T4 is compared with a set of 

 mutants induced by the action of 5-bromouracil. When analyzed by genetic 

 mapping technic}ues of high resolution, the two sets of mutants are found to be 

 quite different. The mutagen does not merely enhance the over-all mutation rate 

 but acts at specific locations in the hereditary structure. 



The induced mutants are mostly of the nature of small, revertible alterations 

 rather than gross defects. The reversion rates of induced mutants are less varied 

 than those of spontaneous mutants, indicating that a certain class of molecular 

 transition is involved. 



These preliminary results encourage the hope that this sort of genetic analysis 

 can lead toward an understanding of the mechanism of mutation and the identifica- 

 tion of the specific chemical configurations composing the genetic structure. 



Note: The occurrence at a given location of one mutation in the induced set 

 corresponds to roughly 10^ limes higher mulahility of this spot in 5-bromouracil than 

 is indicated by the occurrence of one mutation in the spontaneous set. 



* Aided by grants from the National Science Foundation and the American Cancer Society 



t Present address: Medical Research Council for Molecular Biology, Cavendish Laboratorj^, 

 Cambridge University, England. 



X Research Fellow of the Damon Runyon Memorial Fund for Cancer Research. Present 

 address: Biological Laboratories, Harvard University, Cambridge, Massachusetts. 



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'* In order to minimize the background of r mutants present in the stock of standard type, which 

 is usually around 2 X 10 ~^, several stocks of T4 were grown, and one with a small proportion of r 

 mutants was selected. In the stock used, this proportion was less than 10 ~^. 



15 S. S. Cohen and H. D. Barner, J. Bacterial, 71, 588, 1956. 



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