IMMUNITY AND PROPHAGE LOCALIZATION 513 



TABLE 2 



Occurrence of Wild Type Recombinants with the Immune Specificity ok 

 X IN Crosses between a Series of X Mutants and Three Related Phages 



" Recombinants arose which were more turbid than Xcoi, but less turbid than 

 Xcoi"*". 



Key: + signifies the occurrence of wild type recombinants with the immune 

 specificity of X; — indicates failure to detect wild type recombinants among 1000 

 offspring with the immune specificity of X. 



Note: Several mutants of X (X rui, X cOi, X Ci, X coi, andX mi) were crossed to 434 

 and the phage progeny was plated on K12(434), where plaques with the wild 

 type morphology and the immunity specificity of X could be detected. Analogous 

 crosses were carried out with phages 21 and 82. 



C42, and COl, from region II. A map showing the positions of the clear 

 mutants employed is given in Fig. 1. 



The crosses made were 434 X XCi, where Cx represents individual 

 members of the selected series of clear mutants. The progeny of each 

 cross were plated on K12(434) and on K12(X). The presence of turbid 

 plaques on K12(434) demonstrates the presence in 434 of a Cx^ allele, 

 which can recombine with the locus controlling immunity specificity of 

 X. Clear plaques on K12(X) reveal phages with the immunity of 434 

 which have picked up a Cx allele from X. A comparison of the proportion 

 of turbid plaques on K12(434) with the proportion of clear plaques on 

 Kr2(X) serves as an internal control, because they are reciprocal re- 

 combinants and should therefore have the same frequency. When 434 

 alone is plated on K12(X), there are 0.07 % clear plaques due to mutants 

 in the 434 stock. This proportion is, naturally, present as a background 

 in the yield of all of the crosses. The proportion of turbid mutants in a 

 stock of clear X is less than 1 in 10* and is therefore too low to be de- 

 tected in the crosses. 



The results of this series of crosses, given in Table 3, show that 

 recombinants arise from the crosses with mutants Cu, C42, and coi, but 

 for none of the others. There is, therefore, recombination between 434 

 and X in regions II and III of the c segment but not in region I. The 



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