344 F. JACOB AND E. L. WOLLMAN 



C. Specific Locations of Different Prophages 



The fact that, whereas a lysogenic bacterium most frequently carries only 

 one prophage of a given type, it may carry several unrelated prophages, 

 suggests that there exists a specific bacterial site for each type of prophage. 

 The location of fourteen different prophages has been investigated by means 

 of E. coli K12 crosses (Jacob and WoUman, 1957). 



Half of these prophages are UV-inducible. Although each of them exhibits 

 a different immunity pattern, the homologous phages appear to be serologic- 

 ally related and able to undergo genetic recombination on mixed infection of 

 sensitive bacteria. All these UV-inducible prophages have been found to be 

 linearly arranged on a segment of the bacterial chromosome close to the Gal 

 character. On this segment, each of the prophages occupies a definite location. 

 Moreover, in crosses Hfr ly~^ X F~ ly~, zygotic induction is observed with 

 every prophage, although at different degrees depending on their specific 

 location. 



On the contrary, UV-noninducible prophages appear to be located on other 

 segments of the bacterial chromosome. No zygotic induction occurs with any 

 of them. 



It can therefore be concluded that each of these fourteen prophages 

 occupies a specific location on the chromosome of E. coli K12. Ability to 

 lysogenize thus appears to involve a genetic property of the phage, as well 

 as the existence on the chromosome of the host of a specific site for the 

 homologous prophage. Mutations of the host could thus be imagined which 

 would involve the loss of its ability to be lysogenized. Such mutations have 

 not as yet been observed. 



Work on doubly or triply lysogenic strains for phage P2 has led Bertani 

 (1956) to conclude that noninducible prophage P2 could occupy, with equal 

 probabilities, one of three different locations. 



From the locations found for inducible prophages on the one hand, for 

 noninducible ones on the other hand, UV inducibility could be a property, 

 not of a given phage as such, but perhaps of the chromosomal segment on 

 which certain types of prophages happen to have their location. 



D. Nature of the Prophage 



Our knowledge about the chemical nature of the prophage or its physical 

 relationship to the bacterial genetic apparatus is stO] a matter of speculation. 



Since lysogenic bacteria obtained after infection with a temperate phage 

 produce phage particles identical with the original type, the prophage must 

 have the same genetic potentialities as the original phage. From experiments 

 by Hershey and Chase (1952), it is apparent that, in phage reproduction, 

 these potentiahties are borne by the DNA of the infecting particle. Several 



