382 F. W. STAHL 



(1957) showed that this can be the direct consequence of host-cell reactiva- 

 tion. They found a considerable increase in multiplicity reactivation of Tl 

 on cells which had lost reactivating power as a result of ultraviolet irradia- 

 tion. 



Two not mutually exclusive explanations for the small amount of multipH- 

 city or cross reactivation in systems showing host-cell reactivation can be 

 offered: (1) The reactivation mechanism works in such a way that a fraction 

 of the particles are completely reactivated while the number of hits on the 

 remaining particles is not reduced at aU. (A recombinational mechanism of 

 reactivation might be expected to have this consequence.) As a result, the 

 inactive particles in an irradiated phage stock have a higher number of hits 

 than would be calculated from the fraction of survivors assuming a Poisson dis- 

 tribution of hits. They would, therefore, show an apparently reduced abihty 

 to be cross or multiphcity reactivated relative to T-even phage. (2) Vegeta- 

 tive phages in systems showing host-cell reactivation tend to mate with their 

 homolog in the host ceU rather than with each other. This would lead to a 

 reduction in the amount of phage-phage reactivation, which depends, as we 

 have seen, on recombination between the phages. In line with this view is 

 the observation that with such phages the number of rounds of mating in 

 normal crosses is less than with the T-even phages (see Stent, 1958, for an 

 extended discussion of this point). 



D. The Induction of Mutations by Radiation 



There has been no report of an increased mutation frequency in T-even 

 phages among the survivors of ultraviolet irradiation. However, increased 

 mutation frequencies have been observed for phage produced by multi- 

 plicity reactivation following ultraviolet irradiation. Latarjet (1949, 1954) 

 irradiated T2-infected cells after the onset of vegetative phage multiplica- 

 tion (after 6 minutes). The fraction of cells surviving the irradiation and 

 liberating host-range mutants increased. Krieg (personal communication) 

 has observed reversions to wild-type of T4 rll mutants. He apphed the 

 ultraviolet light to free phage and observed the progeny produced following 

 multiple infection. It remains to be decided whether the role of multiplicity 

 reactivation in these experiments is simply to permit the observation of 

 progeny from phage which have received a high dose of irradiation. 



The appearance of mutants among the survivors following ultraviolet- 

 irradiation has been reported for a number of other phage systems. In some 

 cases mutation has been observed after irradiation of the host cell only 

 (T3, Fraser, 1957; A, Jacob, 1954). In these cases a more pronounced effect is 

 observed if both the phage and host cell are irradiated ( Weigle and Dulbecco, 

 1953; Weigle, 1953). A possible increase in mutation rate following irradia- 

 tion of the free phage alone has been reported for Tl (Tessman, 1956). In 



