510 KAISER AND JACOB 



property, called immunity, is phage specific since lysogenic bacteria re- 

 main, with some exceptions, sensitive to all other phages that can infect 

 their nonlysogenic counterparts. Immunity is reciprocal, in the sense 

 that a bacterium lysogenic for any temperate mutant of a phage is 

 immune against the wild type and its mutants. Several experiments 

 (Jacob, 1954; Bertani, 1954; Goodgal, 1956) support the idea that the 

 deoxyribonucleic acid (DNA) of homologous phage can penetrate into a 

 lysogenic bacterium, suggesting that immunity is due to a block, which 

 prevents phage multiplication inside the bacterium. 



Little is known about the mechanism of immunity or the origin of its 

 high degree of specificity. It has been suggested that there is a relation 

 between immunity and the specific location occupied by a prophage on 

 the chromosome of its host (Lwoff, 1953). 



A new experimental approach to the problem of immunity was opened 

 when a series of temperate phages, all active on E. coli K12, was isolated. 

 Each of these phages has a different specificity of immunity: bacteria 

 lysogenic for any one of them are still sensitive to the others. Yet, 

 genetic recombination occurs between pairs of these phages when they 

 multiply together in the same bacterium. Thus, the genetic control of 

 immunity could be studied by phage crosses. Not only do these phages 

 differ in their specificity of immunity, but they differ also in their lo- 

 cation as prophages in the host. Each of them appears to occupy a 

 specific location on the bacterial chromosome (Jacob and WoUman, 

 1957). Phage crosses offer, therefore, the possibility of determining which 

 part of the phage genetic material determines the specificity of im- 

 munity and which part determines the specificity of the prophage lo- 

 cation on the bacterial chromosome. The results of such crosses are the 

 substance of this report. 



MATERIALS AND METHODS 



Only those materials and methods which were not described in a 

 previous publication (Kaiser, 1957) will be given here. 



Media. EMB galactose agar contains 10 gm of Bacto-tryptone, 5 

 gm of yeast extract, 2 gm of K2HPO4, 0.4 gm of eosin Y, 0.065 gm of 

 methylene blue, 5 gm of D-galactose, and 15 of gm Bacto-agar per liter 

 of distilled water. 



Bacteria. A galactose negative mutant of E. coli K12 strain C600 was 

 selected by the penicillin method (Davis, 1948; Lederberg and Zinder, 

 1948) following an irradiation with ultraviolet light. This mutant is 



354 



