RADIOBIOLOGY OF BACTERIOPHAGE 371 



B. Comparative Radiobiology of T2 and Ti 

 1. Photoreactivation 



a. PJiotoreactivable Sector. It was observed by Dulbecco (1949) that the 

 infectivity of a fraction of the inactive particles in an irradiated phage 

 suspension could be restored by shining visible light on cells to which the 

 inactive phage had adsorbed. The degree to which photoreactivation can 

 occur differs among the T-phages; of the T-even phages, T2 shows the greatest 

 degree of photoreactivation and has been the principal object of study. A 

 survival curve for infectivity under conditions of maximal photoreactivation 

 can be obtained. Such a curve is essentially identical to the survival curve 

 obtained without photoreactivation, except that the slope is reduced. The 

 fraction by which the slope is reduced is a characteristic of the phage and is 

 referred to as the "photoreactivable sector." The photoreactivable sector 

 for T2 is 0.56 and for T4 is 0.2 (Dulbecco, 1950). 



Photoreactivation could be interpreted in two general ways; within these 

 interpretations a variety of specific schemes is possible. (1) Visible light 

 performs some function which in the dark must be carried out by the phage 

 particle. (2) Visible light erases a fraction of the hits in the phage population. 

 As pointed out by Bowen (1953), the widespread occcurrence of photo- 

 reactivation makes the second interpretation far more likely. 



An experiment by Lennox et al. (1954) is strongly in support of the idea 

 of erasure of ultraviolet-induced damage. They irradiated two different 

 populations of infected cells and measured their ability to produce phage. 

 One population was irradiated and then photoreactivated; the other popula- 

 tion had been irradiated to the survival level of the first population following 

 its photoreactivation. The sensitivities of these populations to additional 

 ultraviolet irradiation was found to be identical. 



b. Kinetics of Photoreactivation. The restoration of infectivity as a function 

 of dose of reactivating light has been examined by Dulbecco (1950 and 1955) 

 and Bowen (1952). With phage T3, reactivation shows the expected "multi- 

 hit" kinetics; the number of quanta required to restore infectivity to a 

 reactivable particle is equal to the nimiber of hits which the particle has 

 received (Dulbecco, 1955). For T2, however, the results are puzzhng. The 

 T2 preparations used by Bowen had received on the average about 7 hits. 

 Since the photoreactivable sector for T2 is 0.6, about 4 hits per particle must 

 be thought of as being "reactivable." He found, however, that to a good 

 approximation the restoration of infectivity responds to visible light as if 

 only one quantum of light were required to reactivate a given phage particle. 

 Bowen has indicated (1952) how this finding might be reconciled with the 

 idea of erasure of damage. He suggests that reactivable hits are of several 

 classes. The frequency and quantum efficiency for the restoration of each 



