LYSOGENY 343 



appears to be linked with another genetic character which controls galactose 

 fermentation (Gal). Such results support the hypothesis that the lysogenic 

 character is under the control of a nuclear determinant. That this deter- 

 minant is the prophage itself appears from experiments involving two lyso- 

 genic strains, each one carrying a different A mutant. As shown by Appleyard 

 (1954a), in such a cross the genetic markers of the prophage itself appear to 

 be linked to the Gal character. However, in certain crosses, anomahes were 

 observed in the inheritance of the ly and Gal characters (WoUman, 1953; 

 Appleyard, 1954a; Fredericq, 1954). 



Recent advances in our knowledge concerning bacterial recombination 

 (WoUman et al., 1956) have allowed a more accurate investigation of the 

 genetic determinism of lysogeny (Jacob and Wollman, 1957). In E. coli 

 K12, bacterial conjugation may be visualized as the oriented injection of a 

 linear segment carrying Hnked genes from donor (or Hfr) into recipient (or 

 F~) bacteria. In crosses between nonlysogenic donor Hfr ly~ and lysogenic 

 recipient F~?2/+ the nonlysogenic character is transferred and segregates 

 among recombinants as any other genetic character. It can be located at 

 about 15 recombination units from Gal. 



In crosses where both donor and recipient parents are lysogenic, each one 

 carrying a different A mutant, the Hfr prophage segregates among recombin- 

 ants exactly as the nonlysogenic character in the previous cross. The con- 

 clusions can therefore be drawn that A prophage occupies a definite position 

 on the bacterial chromosome. In the two crosses previously mentioned, it 

 behaves exactly as any bacterial genetic character. 



However, the situation is quite different in crosses between Hfr ly^ and 

 V~ly~. The ly^ character of the donor is not transmitted to recombinants. 

 This is due to the fact that whenever a donor injects into a nonlysogenic 

 recipient a segment of chromosome carrying the A prophage, this prophage 

 develops in the recipient ceU. The recipient cell lyses and releases one 

 hundred phage particles. This phenomenon is called zygotic induction. When- 

 ever A prophage is transferred to a nonlysogenic recipient, it behaves as a 

 lethal character immediately expressed. As a consequence, those zygotes 

 which have received a piece of the Hfr genetic material carrying A are 

 destroyed. Nothing similar can be demonstrated in crosses involving a lyso- 

 genic r~ parent, whether or not the Hfr parent is lysogenic. The presence 

 of a prophage confers to the F" cell an immunity against zygotic 

 induction. 



Thus, experiments of bacterial recombination demonstrate that lysogeny 

 is under the control of a nuclear determinant, the prophage itself, which is 

 located at a specific site of the bacterial chromosome. However, the pheno- 

 menon of zygotic induction indicates that the prophage does not behave 

 exactly as a normal genetic component of the host. 



