1 LURIA 



exposed to ultraviolet light and "inactivated" — in the sense that infec- 

 tion of a bacterium with one such irradiated particle, while killing the 

 bacterium, would not cause any phage production (24, 25). For re- 

 activation to take place, the inactive infecting particles must be of the 

 same type or of genetically related types (T2, T4, T6). This can be 

 interpreted on the basis that reactivation is due to replacement of dam- 

 aged portions or units of the genetic material of one infecting particle 

 by homologous undamaged portions supplied by the other particles, 

 by the same (unknown) mechanism responsible for the genetic re- 

 combinations discussed above. This interpretation of multiplicity re- 

 activation leads to specific expectations concerning the frequency with 

 which bacteria infected wdth inactive phage particles should produce 

 active phage. On the one hand, the greater the dose of radiation, the 

 smaller should be the frequency of the bacteria that receive two or more 

 particles which can successfully supplement one another, because of 

 more frequent damage in homologous genetic units. On the other hand, 

 for a given dose of radiation, the frequency of bacteria producing active 

 phage should increase with increasing "multiplicity of infection," since 

 this increases the fraction of bacteria that contain mutually supplement- 

 ing groups of inactive particles. Both expectations are borne out by 

 experiment, and the results agree reasonably well with quantitative 

 expectations derived from a mathematical rationalization of the genetic 

 hypothesis of reactivation. This hypothesis, however, should be con- 

 sidered simply as a working hypothesis until it is substantiated by 

 independent evidence; for the time being it rests mainly on analogy 

 and on a mathematical analysis involving several unproved assump- 

 tions. Dulbecco (10, 11) has recently discovered in my laboratory 

 that ultraviolet inactivated phage attached to its host bacterium can be 

 reactivated by exposure to visible light ("photoreactivation," 20). The 

 results of this work may affect, in a way that is not yet clear, the 

 interpretation of multiplicity reactivation as well. 



Be this as it may, the interactions among phage types in mixed in- 

 fection indicate that in phage reproduction specificity is perpetuated 

 not as a whole, but subdivided into discrete units; we must then look 

 upon these units as the elements whose specific structure is replicated. 

 This result alone does not prove, however, that the units are replicated 

 separately: one could imagine that, after the initial "eclipse," each new 

 phage particle is produced as a whole and that all recombinations result 

 from late interactions among the newly formed particles. To gain in- 

 formation on this point let us return to the experiments on the kinetics 

 of intracellular phage production. 



We saw above that active phage particles appear only around the 



