114 S. E. LURIA AND R. DULBECCO 



sponded to putting ^ = 2 in formula (6) and to comparing the experimental val- 

 ues of w with the values of y for x = 0. Upon closer analysis, this method proved 

 incorrect, because the contribution of higher multiple infection cannot be 

 neglected, even for low multiplicities. Analysis of more data showed that the 

 probability of reactivation is in fact not constant for low values of x, but ap- 

 pears to be so for low doses, because the differences are small and of the order 

 of the experimental errors. The use of the wrong approximation made our 

 previous estimates of n too high. 



Yield of active phage from bacteria in which reactivation occurs 



The yield of active phage following reactivation was studied systematically 

 for phages T2r and T4. After infection, bacteria were diluted and allowed to 

 lyse in liquid, as in a typical "one-step growth" experiment. The latent period 

 before lysis is somewhat longer than for active phage (about 26 minutes instead 

 of 21 for T2, 30 minutes instead of 25 for T4), and the rise in phage titer upon 

 liberation somewhat slower. All yields were calculated from plaque counts after 

 the titer had reached a steady level. The results, shown in table 6, indicate 

 that the yields are generally somewhat lower than those from bacteria infected 

 with active phage particles. No clear relation of yield to dose of radiation or 

 to probability of reactivation was detected. For T2r irradiated with high doses, 

 there is a certain tendency toward higher yields for higher multiplicities. 



Mixed infection with active and inactive phage 



Transfer of genetic material involved in reactivation must occur between 

 active and inactive phage particles, since, as we saw before, an active particle 

 of a T-even phage can reactivate an inactive particle of another T-even phage. 

 If transfer occurred by reciprocal exchanges of genetic material, we should 

 expect that upon mixed infection with active and inactive particles of the 

 same phage some of the active particles would receive inactive units and, 

 therefore, be inactivated. This possibility was tested for phages T2 and T4 by 

 experiments of the following type. 



Bacteria are added to mixtures containing various proportions of active 

 phage and of phage of the same strain irradiated with different doses. For 

 each mixture, the average numbers of active and of inactive particles adsorbed 

 per bacterium are calculated and, hence, the number of bacteria receiving both 

 active and inactive phage. A plaque count before lysis gives the number of 

 bacteria that liberate phage, while a plaque count after lysis gives the yield of 

 phage per bacterium. In this manner, we can determine whether inactive phage 

 suppresses production of active phage from bacteria that also adsorb an active 

 particle, or possibly affects the yield. 



The results of these tests can be listed as follows: 



(a) a bacterium receiving an active particle plus one inactive particle of 

 the same phage — no matter how many hits the latter has received — never fails 

 to liberate active phage; 



(b) part of the bacteria that adsorb one active particle plus several inactive 



268 



