314 BACTERIOPHAGES 



Weigle (1953). The mutation scored by Jacob occurred spon- 

 taneously with a frccjucncy of about 1()~'' per infected bacterium 

 in strain K12S infected with a lambda derivative. The muta- 

 tion frequency was increased by increasing doses of ultraviolet 

 light applied to the host bacteria before infection, reaching a 

 maximum of about 5 X 10~^ per infected bacterium. Similar 

 results were obtained by treating the bacteria with nitrogen mus- 

 tard before infection with phage. There was a large increase in 

 the absolute numbers of mutant phage particles as well as in 

 their proportion in the population, that is, treatment of the host 

 bacteria with the mutagen before infection did not cause an 

 appreciable loss of infecting phage particles. The mutagenic 

 capacity of the treated bacteria varied with the interval between 

 irradiation and infection. Incubation at 37 ° C. increased muta- 

 genic capacity to a maximum after 20 to 30 minutes and then 

 decreased it. 



Thus Weigie (1953) and Weigle and Dulbecco (1953) ob- 

 served a mutagenic effect of ultraviolet light in lambda and T3 

 that required irradiation of both bacteria and phage, whereas 

 Fraser (Hershey, Garen, Fraser, and Hudis, 1954) and Jacob 

 (1 954c) obtained mutagenic effects in the same phages by irradiat- 

 ing only the bacteria before infection. All agree that the 

 bacteria must be irradiated. All the results can be interpreted 

 in terms of the familiar, if complicated, phenomenon of radio- 

 chemical mutagenesis. The interesting alternative mentioned 

 above is genetic recombination between bacteria and phage. 

 The discovery that irradiation of the phages increases frequency 

 of genetic recombination in ordinary phage crosses (Jacob and 

 Wollman, 1955), adds to this argument. 



Further evidence for a possible genetic homology between 

 phage and host cell was obtained by Garen and Zinder (1955). 

 Their evidence indicates that the sensitivity of phages to inacti- 

 vation by ultraviolet irradiation places them in two distinct cate- 

 gories; sensitive phages such as T2, T4, and T6 and resistant 

 phages such as Tl, T3, and P22. The authors suggest that the 

 relative resistance of the latter group is due to replacement of 



