358 r. w. STAHL 



commiinication), however, consistently get one-hit survival curves with T4 

 which has been extensively purified by differential centrifugation. This 

 suggests that ultraviolet light may have a reactivatmg effect on a fraction 

 of the particles in the stock which have been rendered inactive by some 

 "impurity" in the preparation. If such is the case, it must be assumed that 

 the impurity is associated with the DNA of the phage, since phage irradiated 

 after injection of the DNA into the host cell may also show a shoulder (see, 

 for instance, Lennox et al., 1954). Whatever its origin, the shoulder is 

 sufficiently small that for most purposes it can be neglected. 



2. Multiplicity Reactivation 



a. Definition and the Luria Hypothesis. In practice, an additional anomaly 

 in the survival curve of T4 is encountered at high doses; the plaque-forming 

 ability seems to become relatively resistant to ultraviolet light. Luria 

 (1947) showed that this is the result of the infection of the same bacterial 

 cells on the assay plate by two inactive particles. The inactive particles are 

 able to cooperate to produce active phage, and hence a plaque. Luria (1947) 

 suggested the following interpretation of this cooperation between inactive 

 particles (multiphcity reactivation). Each phage particle, he supposed, is 

 composed of a number, n, of equally sensitive units. Each unit is different 

 from the others, and a bacterium will produce phage only if it is infected by 

 particles which among themselves possess a complete set of undamaged 

 imits. For cells infected with exactly k phages, the probability of yielding 

 phage is given by the expression 



[1 - (1 - e-^'")^"]". (5) 



We shall see that this expression is not a sufficient description of the require- 

 ments for multiphcity reactivation in T4. However, the notion of units 

 within a phage each of which is ultraviolet-sensitive marks the beginning 

 of an important phase in radiobiology of phage. This is the notion, explicit 

 in the Luria hypothesis, that the survival curve for single phage particles 

 might meaningfully be written as 



where the/'s are the fractions of the total phage-lethal hits which inactivate 

 target 1, target 2, . . . target n. The multiphcity reactivation experiments 

 tried to count the number n of targets. Subsequent work has tried and in 

 part succeeded in determining the nature of the various targets. Thus, the 

 experiments and the hjrpothesis of Luria were the stimulus for a line of 

 investigation which is now beginning to contribute information on the 

 structure and functional properties of phage. 



h. The Test of the Luria Hypothesis. In practice one cannot generally work 

 with a population of cells all of which have been infected by the same 



