RADIOBIOLOGY OF BACTERIOPHAGE 373 



about 0.6 for T2 and 0.2 for T4. (3) In multiplicity reactivation experiments, 

 the slope at high dose is about 0.2 that of whole phage for T2 and 0.4 for 

 T4.^ (4) In cross-reactivation experiments, with T2m+ phage as the un- 

 irradiated parent, the initial slope for marker knockout (log survival versus 

 minutes of irradiation) is about twice as steep for T2w+ as for T2w, whereas 

 at higher doses the curves are identical for the two phages (Harm, personal 

 communication). (5) In intracellular irradiation experiments (see Section 

 Clb) the sensitivity of T4 (or T2m) does not change for the first 5 to 6 minutes 

 of the infectious cycle, whereas the sensitivity of T2w decreases almost two- 

 fold during this period (Streisinger, 1956; Symonds and McCloy, 1958). This 

 change in sensitivity of T2m+ disappears under conditions of maximum 

 photoreactivation (Symonds and McCloy, 1958). 



We offer these observations without comment except to say that they do 

 not all fit easily into our radiological model for T4. 



C. Irradiation of Infected Cells 



1 . Luria-Latarjet Experiment 



a. Purpose of the Experiment and Results ivith T7. In some phage systems, 

 the ability of the host cell to produce phage upon subsequent infection is not 

 appreciably impaired by irradiation (e.g., see Anderson, 1948). In such cases, 

 moreover, the ability of the cell to produce phage is inactivated at approxi- 

 mately the same rate as are free phage particles if the irradiation is given to 

 singly infected cells inmiediately following infection. This situation makes 

 it possible to study the evolution of the infectious process by determining 

 the changes in sensitivity of the infected cells (Luria and Latarjet, 1947). 

 This potentiality is best illustrated by a discussion of a simple case. 



One may imagine that the only changes in sensitivity which occur arise 

 as a result of multiplication of the infecting phage. In the absence of inter- 

 actions between the new particles (i.e., in the absence of multiplicity reactiva- 

 tion), the sensitivity of the complexes would change in a very simple way; 



^ The significance of this comparison between T2 and T4 is somewhat doubtful. 

 Harm, working with T4 (1956), assumed that the ability of phage to participate in 

 multiplicity reactivation was independent of dose; Dulbecco, working with T2 (1952a), 

 assumed that particles which had lost the ability to kill bacteria upon single infection 

 had lost the ability to participate in multiplicity reactivation. Each calculated x, the 

 multiplicity of infection, accordingly. 



In T2, one hit per 120 inactivates bacterial killing ability (Dulbecco, 1952b), whereas 

 in T4 one hit per 50 is effective (Doermann et al., 1955). Thus, if Harm's assumption is 

 the correct one, the final slopes for T2 and T4 retain most of their difference. If Dulbecco's 

 assumption is correct, however, the difference in final slope for T2 and T4 may be very 

 small. We have seen that the rate at w hich ultraviolet light destroys the ability of T4 

 to participate in cross reactivation is very small. This suggests that Harm's assumption 

 is the correct one. 



