334 BACTERIOPHAGES 



and Tlhr'^, both phage particles are converted to vegetative 

 phage by injection of their nucleic acid, and replication of each 

 begins. Even though the sites of infection may be situated at 

 opposite ends of the bacterial cell the genetic evidence indicates 

 that the two pools of replicating phage DNA soon coalesce and 

 become a single pool. The experiments of Doermann (1953) 

 demonstrated that premature lysates of mixedly infected bac- 

 teria contained almost the same proportion of recombinant 

 phage particles at the beginning of phage maturation as at the 

 time of normal lysis. Genetic recombination must have oc- 

 curred at about the same frequency in the DNA pool before the 

 beginning of maturation as after, and hence the infecting phage 

 particles must have formed a single genetically mixed pool 

 before maturation began. Similar conclusions were drawn 

 from superinfection experiments by Visconti and Garen (1953). 

 The bacteria were singly infected with T2hr phage and after 

 incubation for 6 or 8 minutes were superinfected with an input 

 ratio of 1500 T2A"^r'^ phage particles per bacterium (Chapter 

 XVII). Samples were removed at intervals and diluted into 

 cyanide to stop further phage multiplication and to induce pre- 

 mature lysis. The phage progeny were then assayed for the 

 presence of the genetic markers introduced by the superinfecting 

 phage and present in recombinant phage particles. It was 

 found that the superinfecting parental type and both possible 

 recombinant types were present as mature phage particles as 

 early as 12 minutes after primary infection and 4 minutes after 

 superinfection. The numbers of the complementary recombi- 

 nant types increased during the latent period in the same way as 

 they would have under conditions of simultaneous mixed infec- 

 tion. Apparently the pool of vegetative phage was established 

 by the primary infection and the superinfecting phage then 

 entered this pool as much as 6 or 8 minutes after the primary 

 infection. The superinfecting phage nucleic acid could enter 

 the pool, replicate, undergo genetic recombination, and re- 

 appear in mature phage within 4 minutes after adsorption to the 

 host cell. Such a result would be difficult to visualize if each 



