358 BACTERIOPHAGES 



particle when absorbed to a host cell fails to reproduce itself. 

 However, as we have seen, it is far from inert physiologically. 

 It may kill the host cell, interfere with the multiplication of 

 other phages, or with high intensity visible light undergo photo- 

 reactivation and so regain its ability to reproduce. In multi- 

 plicity reactivation, if two or more inactivated phage particles 

 are absorbed to the same host cell, there is a high probability 

 that this multiply-infected bacterium will lyse and liberate viable 

 phage particles. The probability of multiplicity reactivation in 

 multiply-infected bacteria decreases with increasing ultraviolet 

 dosage but is essentially one with low doses. The probability 

 increases with increasing multiplicity so that an increase in 

 multiplicity can to some extent overcome the effect of an increase 

 in ultraviolet dosage. Multiplicity reactivation is a highly 

 efficient and readily demonstrated property of phages T2, 

 T4, T6, and T5, but is barely detectable with Tl and does not 

 occur with T3 and T7. Inactivated T2 may also display mul- 

 tiplicity reactivation in mixed infection with either inactive 

 T4 or inactive T6, but not with inactive T5. The evidence 

 summarized by Bowen (1953) suggests that the major damage 

 involves phage nucleic acid rather than phage protein and 

 hence may be genetic damage. Luria (1947) proposed a 

 hypothesis to explain these results, assuming ( 7) the inactivation 

 of phage by ultraviolet is due to lethal mutations in a specified 

 number of genetic "units" of the phage particle and (2) that 

 multiplicity reactivation is the result of the recombination 

 of an undamaged unit of each kind to recreate one active phage 

 particle. In order to explain the high efficiency of multiplicity 

 reactivation it was suggested that the undamaged units are 

 capable of independent multiplication before reassembly. 

 Luria and Dulbecco (1949) presented a mathematical formula- 

 lion of the expected survival based on the following simplifying 

 assumptions: (7) every kind of phage contains a fixed number 

 of genetic units, all equally sensitive to radiation; (2) one 

 undamaged representative of each unit is necessary and suffi- 

 cient to give viable progeny; (3) there is a Poisson distribution 



