516 KAISER AND JACOB 



tively, in the two regions. The absence of a significant difference in 

 recombination frequencies suggests that 434hy is homologous to X. 

 Since the Ci region is only about 1 % long, absence of homology there 

 would not be expected to depress recombination frecjuencies significantly 

 in this cross. 



Bacteria lysogenic for 434hy behave exactly like K12(434) in response 

 to infection by X, 434, Xc, 434 c, and434hy. Thus the 434hy prophage 

 determines the immunity pattern characteristic of the wild type 434. 



The existence of hybrids with a Ci region from 434 and the rest of their 

 chromosome from X, which have the specific immunity pattern of 434 

 but not that of X, demonstrates that the Ci region (or a portion of it) 

 controls the specificity of immunity. 



Wild type T4 is capable of multiplication on K12(X), but the rn 

 mutants of T4 are not (Benzer, 1955). T4rii was tested on K12(434) and 

 on K12(434hy); it was found to multiply on both strains. T4r+ was 

 able to multiply on both also. This suggests that the block in T4rii 

 multiplication is caused by the Ci region of X since replacement of the 

 Ci region of X by the Ci region of 434 eliminates the block. 



3. Chromosomal localization of 434hy prophage in E. coli K12. Prophage 

 X and prophage 434 occupy distinct but closely linked loci on the linkage 

 map of E. coli K12 near certain galactose loci (Jacob and Wollman, 

 1957). It must, therefore, be determined whether prophage 434hy oc- 

 cupies the X locus, the 434 locus, or neither. 



In K12(X) prophage X occupies the X locus and in K12(434) prophage 

 434 occupies the 434 locus, by definition. Allelism tests between 

 K12(434hy) and K12(X), and between K12(434hy) and K12(434), 

 should localize the 434hy prophage. 



Allelism tests were performed by means of transduction with phage 

 PI (Lennox, 1955). The occurrence of transductants nonlysogenic for 

 the prophage of the donor and the prophage of the recipient would 

 indicate that the two prophages are nonallelic. The absence of non- 

 lysogenic transductants indicates allelism, subject of course to the limita- 

 tion in the number of transductants examined. 



In the experiments to be reported, a stock of Pike (Lennox, 1955) was 

 prepared on a gal+ donor strain. The gal~ recipient was infected with 

 this Pike stock and gal+ transductants isolated, purified, and tested for 

 prophage by replica plating on C600, C600(X), and C600(434). The 

 recipient strains had all been rendered /X/434 to prevent infection by 

 free phage X or 434. 



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