G. S. STENT AND C. R. FUERST 451 



ciency. Results similar to those presented in Fig. 4 have also been obtained 

 after infection of bacteria with radioactive T3 and X phages. 



Stale of the Phage after Decay. — 



Cross-Reactivation. — The lethal damage sustained by the phage upon decay 

 of one of its phosphorus atoms thus appears to prevent a step of the reproduc- 

 tive cycle which occurs after the invasion of the host. In accordance with this 

 view, we observed that T2 particles inactivated by P^^ decay are still adsorbed 

 to bacterial cells. In fact, such phages are still able to participate in the re- 

 productive processes occurring inside bacteria infected with a normal, non- 

 inactivated related phage. In experiments already presented elsewhere (Stent, 

 1953 h) it was found that a radioactive stock of the double mutant strain 

 Tlhri could still contribute its genetic markers to the progeny of a cross with 

 non-radioactive wild type T2+-1- after P^^ decay had destroyed the ability 

 of the Tlhri particles to reproduce themselves in solo {cross-reactivation). 

 It appeared, furthermore, that the ability of a radioactive T2 particle to donate 

 either one of these two unlinked loci h and ri is destroyed separately by P^^ 

 decay, each locus disappearing at about one-third the rate of the plaque- 

 forming ability of the whole particle. In those infected bacteria in which only 

 one of the two radioactive loci has been inactivated, the surviving locus 

 appears among the progeny in nearly normal yield. Stahl (1954) also discovered 

 the existence of cross-reactivation of genetic markers after inactivation of T4 

 phage by P^^ decay. Stahl observed, furthermore, that the likelihood that a 

 P^- disintegration prevents both of two markers from appearing among the 

 progeny of a cross with an active phage is inversely related to the genetic 

 linkage distance of their loci. Hence it may be inferred that the lethal damage 

 of P'^ decay affects the reproduction of only part of the hereditary substance 

 of the bacteriophage particle, leaving the rest intact to reproduce itself in mixed 

 infection with an active phage. 



Multiplicity Reactivation.- — The presence of an active phage particle in the 

 same bacterial cell, however, appears to be necessary for the survival of the 

 undamaged parts of a P^^-inactivated T2 phage. Contrary to ultraviolet-in- 

 activated T2 (Luria, 1947), infection of one bacterium by several P^'-in- 

 activated particles does not lead to the production of active phage {multi- 

 plicity reactivation) . 



In order to test for multiplicity reactivation following P^- decay, the stock of radio- 

 active T2 employed in the experiment presented in Fig. 4 was used to infect B/r 

 bacteria at a multiplicity of 2.2 phage particles per cell. As in the low multiplicity 

 experiment of Fig. 4, the mixture of bacteria and radioactive phage was incubated at 

 37°C. for 2.5 minutes before being frozen, stored at — 196°C., and assayed for surviv- 

 ing infective centers from day to day. At a multiplicity of infection of 2.2, the frac- 

 tion of all infected bacteria to which two or more phages are adsorbed (multicom- 

 plexes) is 0.73. Hence if two or more T2 particles were able to cooperate in the 



290 



