434 BACTERIOPHAGES 



infecting phage and the chromosome of the infected bacterium or 

 may consist in a synapsis of phage chromosome with bacterial 

 chromosom.e to form a heterozygous diploid region which is 

 replicated as such. Some of the evidence for this point of view 

 was presented in Chapters XVI and XIX and will be recapitu- 

 lated here. 



/. In lysogenic bacteria the phage genetic material occupies a 

 definite locus on the bacterial chromosome being linked to 

 recognized bacterial gene loci. For instance, the lambda pro- 

 phage in strain K12 of E. coli is linked to a locus involved in 

 galactose fermentation. 



2. Studies by Bertani (1954) with the temperate coliphage P2 

 suggest that there are several loci available for attachment of the 

 prophage, one site being preferred. Superinfection of a P2 

 lysogenic bacterium with a genetically inarked variant of P2 m.ay 

 result frequently in replacement of the original P2 by the super- 

 infecting P2 at the preferred locus, or less frequently may result 

 in a doubly lysogenic cell with a prophage at each locus. How- 

 ever, superinfection with a different phage, PI, readily giv^es rise 

 to bacteria doubly lysogenic for PI and P2, suggesting that these 

 two prophages have different preferred loci in the host cell, 



3. Similarly it is possible to make a culture of Pseudomonas 

 aeruginosa lysogenic simultaneously for two unrelated phages 1 

 and 8, or for two related phages 4 and 8 (Jacob, 1952c). How- 

 ever, superinfection of a culture lysogenic for phage 8 with a 

 plaque type mutant of phage 8 does not result in either substitu- 

 tion or in doubly lysogenic cells suggesting that there is a unique 

 site for prophage 8 in this bacterial strain and other sites for pro- 

 phages 1 and 4 (Jacob and Wollman, 1953). 



4. In addition there is another type of evidence suggesting 

 direct recombination between the genetic material of the phage 

 and the host cell and occurring with some virulent as well as tem- 

 perate phages. Ultraviolet-inactivated temperate phage lambda 

 is reactivated when adsorbed to ultraviolet-treated host cells. 

 Among the reactivated phage particles are found a high propor- 

 tion (as much as 5 to 10 per cent under certain conditions) of 

 plaque type mutants. These are found only when both bacteria 



