312 BACTERIOPHAGES 



the mutagenic effect may well occur within the host cell. 

 Further kinetic study of this system is desirable. 



A remarkable example of the induction of mutations in phage 

 was reported by Weigle (1953), who studied effects of ultraviolet 

 light on the temperate phage lambda and its host. He found 

 that the plaque count of ultraviolet-treated phage was much 

 higher when plated on ultraviolet-irradiated bacteria than when 

 plated on normal bacteria. The bacteria could be irradiated 

 before or as long as 30 minutes after adsorption of the phage. 

 Reactivation was also produced by bacteria that had been treated 

 with X-rays and with nitrogen mustard, but not when they had 

 been treated with hydrogen peroxide. Among the offspring 

 of phage particles reactivated in this way were found six dis- 

 tinct kinds of plaque-type mutants but no host-range mutants. 

 The proportion of mutant plaques increased linearly with the 

 dose of ultraviolet light received by the phage and also with the 

 dose received by the bacteria. Treatment of the irradiated 

 bacteria with visible light eliminated the mutagenic effect. 

 Illumination of irradiated bacteria after infection with irradiated 

 phage increased the survival of the phage but no mutants were 

 found. The author concluded that mutations were produced 

 only when both phage and host bacteria had been treated with 

 mutagenic agents. Weigle and Dulbecco (1953) found that the 

 same type of mutagenesis by ultraviolet light is demonstrable with 

 the virulent phage T3, in which case many of the mutants show 

 extended host range. 



In the meantime, Fraser and Dulbecco (1953) had obtained 

 unexpected results in crosses between spontaneous h mutants in 

 T3. Their results suggested, among other things, that multiple 

 mutations occur in this phage with a frequency that is unreason- 

 ably high relative to the frequency of single mutations. Results 

 of this kind have to be interpreted, in general, as evidence of a 

 mutagenic effect that varies from bacterium to bacterium 

 (Bryson and Davidson, 1951). However, this peculiarity has 

 been seen only in mutants of phage T3, and has not been studied 

 in any detail in this phage. 



