216 FRANgOIS JACOB 



if injected in X nonlysogenic bacteria. If it is assumed that, as in the case 

 of bacterial recombination, most of the transduced X prophages develop 

 when transduction is performed at high temperature, such an experi- 

 ment enables one to estimate the fraction of particles of 363 carrying 

 X material as between 10~® and 10~^ This proportion is of the same order 

 of magnitude as the fraction of particles of 363 found to be able to 

 transduce threonine or leucine characters. 



Prophage recombination. Prophage recombination between two geneti- 

 cally different and complementary prophages has been shown to occur 

 in transduction. In such experiments, phage X was used because its 

 genetic system had been investigated extensively (Jacob and Wollman, 

 19546; Kaiser, 1954). All the known markers of X are located on a single 

 linkage group, which has been represented in Fig. 2. When sensitive 

 bacteria are infected with genetically marked phages, some become lyso- 

 genic. The phage particles released by such bacteria and their progeny 

 remain genetically identical to the original particles used for infection. 

 The markers available for the study of X genetics during the vegetative 

 phase can therefore be used for studying the prophage. 



In the experiments described below, three markers have been used: 

 ms (medium-sized plaques), Co (cocarde) and m, (minute plaques). 

 These markers cover most of the known length of the X linkage group; 

 in crosses involving these three markers the two parental and the six 

 recombinant types can be distinguished easily and all are temperate. 

 During the vegetative phase, the recombination frequency is about 7 

 to 9 % between m^ and Co and 4 to 5 % between Co and mi. 



In the experiment reported in Table 6, lysogenic bacteria P678 

 (XnisComf )/X were infected with particles of 363 grown on K12(Xm^Comi). 

 The mixture was plated on galactose medium. After reisolation on galac- 

 tose agar individual colonies were grown in broth and analyzed for the 

 type of phage spontaneously released. It is seen in Table 6 that, of 240 

 colonies, 213 released only the original type XmaCom^. The other 27 can 

 be classified in two groups. In the first group, corresponding to 18 

 colonies (7.5%), one or several of the original prophage markers have 



(P4) 

 nrig rn5 g| s ce, c co mi 



3 1^ 3 L5 1.5 01 5 



Fig. 2. The linkage group of bacteriophage X. Figures indicate percentage of 

 recombinants found in two factor crosses. 



348 



