EADIOBIOLOGY OF BACTERIOPHAGE 377 



For a = to the quantum efficiency (the probability that a decay will 

 cause the inactivation of a phage particle), we see that the average number 

 of hits at time t is 



r=(x.N,{l-e-") (11) 



No is hmited by the requirement that the medium have sufficiently low 

 specific activity to support the growth of bacteria. Therefore, most experi- 

 ments must be carried out over a length of time which permits the decay 

 of the majority of the incorporated atoms, A survival curve is generated by 

 assays of the stored radioactive phage stock at approximately daily intervals 

 over a period of 2 to 4 weeks. The survival curve is an exponential function 

 of r [Equation (3)] (Hershey et al., 1951; Stent and Fuerst, 1955). The suicide 

 of phage following infection is measured by periodic assay of infected cells 

 maintained in liquid nitrogen (Stent, 1953a). 



B. Cross and Multiplicity Reactivation 



Cross reactivation experiments with suiciding phage have been performed 

 with intracellularly inactivated T2 (Stent, 1953a) and with extracellularly 

 inactivated T4 (Stahl, 1956). The intracellular experiments suffer somewhat 

 from fluctuations in the properties of the thawed samples; the extracellular 

 experiments are complicated by a dose-dependent loss of the ability of 

 suiciding phage to inject DNA into the host cell (Tomizawa, personal com- 

 nnmication). The two techniques give results which lead to essentially the 

 same conclusions. As with ultraviolet hght, linked markers are knocked out 

 (by genie hits) with a correlation that depends on the map distance between 

 them (Stahl, 1956). Unlinked markers are inactivated by genie hits in- 

 dependently (Stent, 1953a) or nearly so (Stahl, 1956). However, cross reactiva- 

 tion of markers from suicide phage differs from the results with ultraviolet 

 irradiated phage in two respects: (1) A given phage-lethal P^^ ]^^ j^^g ^ prob- 

 ability of knocking out a marker which is several times that found with 

 ultraviolet light. (2) The burst size of reactivated markers is only slightly 

 less than the burst size of the marker in the zero-dose sample; with ultra- 

 violet light, the burst size of the reactivated marker is depressed to a value 

 of between 1 and 2 at high dose (Doermann et al., 1955). 



In view of the reduction in abihty of suicide phage to inject DNA, it is not 

 surprising that multiphcity reactivation has not been demonstrated for 

 phage which has been permitted to suicide extracellularly. It seems some- 

 what surprising, however, that phage permitted to decay inside the cell 

 also fails to give multiplicity reactivation (Stent and Fuerst, 1955), 



