MUTATIONS OF BACTERIAL VIRUSES 99 



The sensitivity of a series of bacterial mutants to viruses a, a' , 7, and 7' was 

 studied. It was found that resistance to virus a may be brought about in the 

 same bacterial strain by different mutations. These lead to differences in sen- 

 sitivity to the related virus a' and in other physiological properties. 



Bacterial mutations leading to resistance to a mutant virus lead also to re- 

 sistance to the parent virus. 



Bacterial mutations leading to resistance to unrelated viruses generally 

 prove independent; the same mutation can occur in strains with a different 

 history of previous mutations. One exception to the independence of various 

 mutations was found. A bacterial strain resistant to viruses a and a reverted 

 to sensitivity as a consequence of a mutation to resistance to viruses 7 and 7'. 



ACKNOWLEDGMENTS 



Some of the experiments reported in this paper were done in the summer of 

 1944 in the Department of Biology, Vanderbilt University, Nashville, 

 Tennessee. The author expresses his thanks to Vanderbilt University for 

 the hospitality and for a grant-in-aid, and to Dr. M. Delbruck for many help- 

 ful discussions. 



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