316 BACTERIOPHAGES 



parable experiment could be performed only if the bacteria were 

 first irradiated to cause induction of prophage development. 



The example cited also calls for a distinction between phage- 

 bacterium homology, and phage-prophage homology. At the 

 present time this distinction is not generally possible. It can 

 only acquire meaning, in fact, as the nature of prophage integra- 

 tion into the bacterial genome is elucidated (Chapter XIX) on 

 the one hand, and methods are developed for eliminating unsus- 

 pected prophages from bacteria, on the other. 



Mutagenic action of proflavine in bacteria infected with T2 

 was reported by DeMars (1953). The frequency of mutations 

 aff"ecting lysis inhibition was increased at least 10-fold when pro- 

 flavine was added to the culture at the time of infection. There 

 seemed to be no selective eff^ect of proflavine in mixed infection 

 experiments. 



The incorporation of 5-bromouracil into the DNA of T2 

 (Chapter XV) has mutagenic effects of unusual interest (Litman 

 and Pardee, 1956; Litman, 1956). Both plaque-type and host- 

 range mutants are produced. So far it is doubtful whether the 

 mutational eff"ects are directly related to the incorporation of the 

 analogue. However, the system offers new means of attacking 

 problems of mutagenesis. 



In conclusion, numerous examples of induced mutagenesis in 

 phage are known. In several instances, the results can be inter- 

 preted in terms of genetic recombination between test phage 

 and phage-like material carried by the host bacterium, though 

 clear recognition of such instances is limited to work with known 

 lysogenic bacteria. In other instances, as with phage T2, 

 chemical and photochemical mutagenesis seems clear. In such 

 instances, the mutagenic agent usually has to act on the infected 

 bacterium ; action on the extracellular phage particles is ineflfec- 

 tive. In all instances, with trivial exceptions, the bacterium 

 must be exposed to the mutagenic agent. Exposure of the phage 

 may or may not be necessary in addition. 



