INACTIVATION OF VIRUSES 377 



The discoveries in 1947 and 1949 of multiplicity reactivation and 

 photo-reactivation, two phenomena associated with UV inactivation, 

 greatly influenced the direction of UV research. These processes will be 

 treated in detail in another chapter, but should be discussed briefly 

 here because tliey reveal something about the state of UV-inactivated 

 particles: 



Multiplicity reactivation (Luria, 1947; Luria and Dulbecco, 1949; Dulbecco, 

 1952) occurs when two or more UV-damaged phage particles infect the same 

 cell. Under these conditions the chance of that cell producmg normal phage 

 is much higher than one would expect on the basis of the viability of tlie 

 individual infecting particles. Photoreactivation of viruses (Dulbecco, 1949, 

 1950) is observed when a ceU has been infected with a UV-damaged particle 

 and subsequently is exposed to visible light. The action spectrum for the 

 reactivation process has its maximum at about 3500 A; wavelengths under 

 3100 or above 4500 A are almost inefl'ective. 



These reactivation phenomena show that the structural changes caused by 

 moderate doses of UV are remarkably stable; thus, a UV-irradiated phage 

 suspension not only retains its survival level unchanged almost indefinitely, 

 but, what is more, the response to reactivating hght remains unclianged. 

 Quantitative experiments have shown that a weU-defined fraction of the UV 

 damage can be reversed by light, and that a single quantum sufiices to return 

 the damaged site to a functional condition (Bowen, 1953). Furthermore, 

 photoreactivated phage-bacterium complexes were fomid to be sensitive to a 

 second dose of UV to the same extent as nonreactivated complexes (at 

 identical survival levels); this suggests that, once reactivated, a complex is as 

 good as new (Lennox et al., 1954). 



Several other phages and a number of plant viruses have been tested for 

 reactivation after UV inactivation. Fluke (1951) tested phage Tl inactivated 

 by UV of different wavelengths, and found that a constant fraction (about 

 70 %) of the inactivated phage could be reactivated by light. This high degree 

 of photo-reactivability seems to disappear if Tl is irradiated with UV in the 

 dry state (Hill and Rossi, 1952). The dry-irradiated phage was found to 

 adsorb normally onto bacteria and the absence of photo-reactivation has not 

 been explained (Hill and Rossi, 1954). Other coliphages tested are less re- 

 activable than Tl; the closely related phages T2, T4, and T6 differ signifi- 

 cantly in reactivability. Price (1950) and Kleczkowski and Kleczkowski 

 (1953), working with staphylococcus and Rhizobium phages, respectively, 

 observed photo-reactivation but failed to elicit multiplicity reactivation. The 

 reason for this failure is not known; but, considering that two or more par- 

 ticles must reproduce simultaneously in the same cell for reactivation to 

 occur, exclusion phenomena, preventing entry of a second particle, should be 

 considered. 



