EFFECT OF RADIATIONS ON PHAGE PARTICLES 73 



"activated" for the reactivation process. Lennox, Luria, and 

 Benzer (1954) carried out experiments in which phage-bacterium 

 complexes were inactivated by ultraviolet light, photoreactivated 

 and then subjected to a second ultraviolet irradiation. They 

 observed that photoreactivated complexes have the same ul- 

 traviolet sensitivity as nonreactivated complexes and inferred 

 from this fact that photoreactivation probably constitutes a 

 direct reversal of the primary ultraviolet damage, rather than a 

 by-pass mechanism. 



/. Multiplicity Reactivation 



When assaying phage stocks inactivated by ultraviolet ir- 

 radiation, Delbriick and Bailey noticed that the plaque counts of 

 surviving phage particles appear to depend on the relative pro- 

 portions of phage particles and bacteria in the plating mixture. 

 Luria (1947) investigated this titration anomaly further and 

 discovered the phenomenon of multiplicity reactivation. An 

 ultraviolet-inactivated phage particle, though unable to re- 

 produce itself, is, as we have already seen, far from physiologically 

 inert. It may kill the host cell, interfere with the multiplication 

 of other phages, or even regain its ability to reproduce after 

 exposure to visible light. Now if two or more such ultraviolet- 

 inactivated phage particles are adsorbed to the same host bac- 

 terium, then there exists a good chance that they may cooperate 

 in some way so that this multi-infected cell lyses and liberates 

 viable progeny phage particles. This is multiplicity reactivation, 

 which can be demonstrated readily with phages T2, T4, T6, 

 and T5. It is, however, barely or not at all detectable with 

 Tl, T3, T7, and lambda. Luria formulated a theory of multi- 

 plicity reactivation in terms of genetic exchange of undamaged 

 parts between irradiated phage particles, a theory whose essence, 

 though not its particulars, later work has shown to be indeed the 

 most likely explanation of this phenomenon. The detailed 

 results of studies on multiplicity reactivation, as well as those of 

 an associated phenomenon, cross reactivation, will be considered 

 in Chapter XVIII. 



