214 FRANgOIS JACOB 



In another experiment, a preparation of phage 363 was grown on a 

 double lysogenic prototroph, K12(82)+(X)+, and used for infecting non- 

 lysogenic P678/82/X. The colonies having acquired the Gait character 

 were tested for their ability to release phages 82 or X, and some were 

 found to release both. 



As has already been mentioned, in a given experiment, the proportion 

 of transduced clones which are found to be lysogenic for 363 is inde- 

 pendent of the selective marker. This is also true in transduction experi- 

 ments involving the transfer of lysogeny for one or more of the three 

 prophages — 82, X, and 434. The proportion of clones found to be lyso- 

 genic for 363 is the same, whether or not the transduced clones have be- 

 come lysogenic for 82, X, or 434. Since such transduction experiments were 

 performed with a multiplicity of infection lower than one, it is clear that 

 the transfer of lysogeny for 82, X, or 434 is performed through phage 363 

 and is not the result of some kind of "phenotypic mixing." 



Reconstruction experiments show that no appreciable reinfections 

 and lysogenizations with phage 363 occur on the selective plates. It 

 appears therefore that the genetic material of several viruses can be 

 included in a single phage coat. 



In transduction experiments, the transfer of lysogeny has been found 

 to be about as frequent as that of nonlysogeny. This finding is not in 

 agreement with the results found in bacterial recombination. In crosses 

 between lysogenic Hfr(X)+ and nonlysogenic F", X prophage develops 

 in most of the zygotes (Jacob and Wollman, 1954a). This development 

 brings out the destruction of such zygotes which lyse and release phage 

 particles. Lysogenic recombinants are therefore very rare. On the con- 

 trary, little or no development of the prophage is observed in the reverse 

 cross between nonlysogenic Hfr and lysogenic F~(X)+, and the frequency 

 of nonlysogenic recombinants found agrees with expectations. The dis- 

 crepancy between the results obtained by transduction and those ob- 

 tained by bacterial recombination can be due to the fact that trans- 

 duction experiments were performed at 20°, a temperature at which 

 phages 82, X, and 434 do not multiply, whereas bacterial crosses were 

 performed at 37°. It is therefore possible that, at 37°, a fraction of the 

 transduced prophages is able to develop during transduction experi- 

 ments. A temperature effect on the frequency of X-lysogeny transfer 

 was observed by Appleyard (1954a). 



In order to test this possibility, the following experiment was designed 

 (see Table 5). Phage X (as well as 82 or 434) forms plaques with the same 



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