IMMUNITY AND PROPHAGE LOCALIZATION 



515 



by plating on K12(X). Step three consisted in replacing Ca by C44+ by 

 crossing to Xci and picking the turbid recombinants on K12. In step 

 four, 434 C44+ was crossed to XC42. A 434 C44"'"C42 was selected by picking 

 a clear plaque on K12(X). The final step consisted in replacing C42 by 

 Ci2^ by crossing 434 C44+C42 to Xci and plating on K12. At each stage in the 

 backcrosses the immunity pattern was checked. This sequence of 5 

 successive backcrosses would be expected to replace all of the parts of 

 the 434 chromosome capable of recombination with the homologous 

 parts from X. We have, therefore, a phage which should possess the Cj 

 region of 434 embedded in an otherwise X genome. This phage will now 

 be designated 434hy. 



The hybrid character of 434hy is evidenced by the following properties. 

 Apart from its immunity pattern, which is that of 434, phage 434hy 

 behaves like X. On K12 it forms large turbid plaques identical to those of 

 X and it has the host range of X. Whereas phage 434 is inactivated by 

 anti-X serum at a slower rate than is X, the hybrid is inactivated at the 

 rate characteristic of X, as is shown in Fig. 2. Further evidence for the 

 homology of 434hy with X is shown by a comparison of X X 434hy with 

 XXX crosses. In a cross XmsCi X 434hy mi, 12 % recombination was ob- 

 served between ms and Ci and 4 % between Ci and mi. A corresponding 

 cross between Xm^c and Xmi gave 8% and 5% recombination, respec- 



10" 



10' 



Fig. 2. Neutralization was measured by incubating a mixture of phage and a 

 1/100 dilution of rabbit anti-X serum at 37°. At the times indicated aliquots were 

 withdrawn, diluted at least 1/10'' and plated on C600. The surviving fraction is 

 the ratio of the number of plaques at time t to the number at time zero. 



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