MUTATION AND PHENOTYPIC VARIATION IN PHAGES 315 



damaged phage material by recombination with homologous 

 genetic material present in the host cell. Phages that are highly 

 sensitive to inactivation may have no genetic material homol- 

 ogous to that of their host cell, and this is consistent with the 

 gross chemical differences between the DNA of some of these 

 phages and that of their hosts. The hypothesis of replacement 

 is also supported by the fact that irradiation of bacteria before 

 infection depresses their ability to support growth of irradiated 

 P22 but not that of unirradiated P22 or of irradiated T2. 



The general hypothesis of phage-bacterium homology can, 

 however, be placed in a different light, as suggested by the fol- 

 lowing experiments. 



Jacob and Wollman (1953) described the properties of a viru- 

 lent, "inducing" mutant of the temperate phage lambda. This 

 phage plated with equal efficiency on the lysogenic host K12 

 (lambda) and on the nonlysogenic strain K12S. However, 

 ultraviolet-inactivated phage gave a much higher plaque count 

 on host strain K12 (lambda) than on K12S. This reactivation 

 occurred only in lysogenic bacteria carrying prophage lambda, 

 not in bacteria carrying unrelated prophages. When the ir- 

 radiated superinfecting phage carried additional genetic markers, 

 the reactivated phage was predominantly of the superinfecting 

 type, but some bacteria liberated products of recombination 

 with the carried phage. The proportion of recombinants 

 in the phage yield increased with increasing doses of ultraviolet 

 light to the superinfecting phage. Jacob and Wollman inter- 

 preted the recombinants as the result of induction of the develop- 

 ment of prophage by the infecting particles, following by genetic 

 recombination between the two kinds of vegetative phage. 



This example suggests the possibility that apparent muta- 

 genesis could often be the result of genetic recombination between 

 the test phage and an unknown prophage carried by the bacter- 

 ium. The fact that in this example, the effect is produced by irra- 

 diating the phage alone, is uniinportant, since the result depends 

 on the special property of this phage to cause the induction of 

 the carried prophage. With other mutants of lambda, the com- 



