318 BACTERIOPHAGES 



mutagenic agents only when these are applied to the infected 

 bacterium. They arise in a stepwise manner producing mutant 

 subclones, and in most instances prove to carry single mutant 

 loci. These mutations showing expected characteristics have 

 been studied with reasonable care only in T2 and T4. 



In crosses between T2 and T4, both unexpected and expected 

 genetic differences are brought to light. The expected ones 

 consist of a pair of allelic genes responsible for the differences in 

 host range and serological specificity, and another pair respon- 

 sible for the differences in sensitivity to ultraviolet light. The 

 unexpected one is a difference in the DNA-glucose content to 

 which no locus can be assigned, because the gross difference 

 fails to segregate in crosses. This difference is correlated with a 

 difference in host range not affecting adsorption. The origin of 

 a similar difference by mutation has not been observed. 



Other anomalous mutations arise under conditions suggesting 

 special interactions between phage and bacterium, possibly ge- 

 netic recombinations, and possibly involving carried, phagelike 

 materials rather than bacterial chromosomes proper, a distinction 

 that cannot yet be made in most cases. Such mutations have 

 been seen only in T3 and lambda. In T3, the spontaneous 

 mutants arise only under special physiological conditions, and 

 the mutants tend to show multiple-factor differences from wild 

 type. In both T3 and lambda, irradiation of the bacterium 

 before infection increases the frequency of mutation. It is 

 possible, of course, that these are, after all, for the most part 

 photochemically induced mutations of the conventional sort. 

 If so they are no less remarkable, because in this case a clear 

 separation of the inducer, ultraviolet light, from its ultimate 

 target, the phage chromosome, has been achieved. 



The physiological effects of mutational changes present a 

 broad field for biochemical exploration, but this has scarcely 

 been started as yet. The topic will be discussed briefly in 

 Chapter XVIII. 



