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S. BENZER 



[vol. 63 



In experiments with E. coli, strain B, and phage T2, Luria and Latarjet found 

 that immediately after infection the survival curve agreed with that of free 

 phage. However, at later times, instead of showing a progressive increase in 

 multiplicity with constant slope, the curves during the first half of the latent 

 period showed a progressive decrease in slope (i.e., a decrease in sensitivity to 

 ultraviolet) while remaining essentially exponential in character. At mid-latent 

 period the curves became multiple-target in character, and thereafter the 

 sensitivity increased again. Since the results did not resemble the family of curves 



Figure 1. Theoretical survival curves for complexes (after Luria and Latarjet, 1947). 



For an individual target the survival y is given by e~^ , where D is the dose in arbitrary 

 units. 



For a complex containing n identical, independent targets, the survival of any one of 

 which is sufficient for survival of the complex, y = 1 — {1 — e~^)". 



in figure 1, it was not possible to perform a target-theory determination of the 

 number of intracellular phage particles. Latarjet (1948) repeated these studies, 

 using X-rays, and observed changes which were similar in character although 

 differing in degree. 



Further investigation of this problem was suggested by the discovery of 

 "multiplicity reactivation" (Luria, 1947; Luria and Dulbecco, 1949). In this 

 phenomenon, two ultraviolet inactivated phage particles, when infecting the 

 same cell, can combine their resources, leading to the production of active phage. 



299 



