134 BENZER ET AL. 



a long time (hours); if adsorbed on metabolizing bacteria, photore- 

 activability ceases after 20 to 30 minutes (28°C). 



After a sufficient exposure to the photoreactivating light a fraction 

 of the inactive particles is reactivated and this fraction is not increased 

 by further exposure to light. This divides the inactive particles into 

 two classes, photoreactivable and non-photoreactivable ones. The rel- 

 ative size of these two classes is a function of the dose of UV used for 

 inactivation. Survival curves (see 11) of the same phage in darkness 

 and after maximum phtr have similar shape; the straight parts of these 

 two curves (obtained at higher UV doses) make an angle. The ratio 

 of the slope of the survival curve after phtr to the slope of the survival 

 curve in darkness defines the "non-photoreactivable sector b of the 

 cross-section of a phage particle for UV." The "photoreactivable sector" 

 a=i — b is taken as an index of the photoreactivability for different 

 phages. The phages of the T group listed in order of decreasing photo- 

 reactivability are Ti, T2, T6, T7, T3, T4, T5. The reactivable sectors 

 of these phages range from about 0.7 to 0.2. 



Phtr as a function of the time of exposure to a constant photo- 

 reactivating light is a one-hit phenomenon: one quantum of the photo- 

 reactivating light is therefore sufficient for phtr of one phage particle. 

 The rate of phtr is independent of the UV dose used for inactivation; 

 it increases with the light intensity, linearly for low intensities and 

 more slowly at higher intensities, tending to a maximum rate; the rate 

 of phtr increases with increasing temperature, with a Qio varying with 

 the temperature from 8 near 5°C to 1.7 at 37°C. 



Active phage does not show light absorption in the wave length 

 region active for photoreactivation, but after UV irradiation of a puri- 

 fied phage preparation an absorption band appears in this region. The 

 photosensitive pigment involved in the process of phtr might be this 

 unknown substance produced in the phage or it might be a bacterial 

 constituent. 



The kinetics of the phenomenon of phtr supports the view that the 

 reactivating light dissociates an inhibiting molecule produced by UV 

 treatment in a phage particle, blocking some essential function, and 

 that the dissociated inhibitor is then destroyed by a bacterial enz3ane. 



A very slight amount of phtr has been observed in phage inacti- 

 vated with X-rays. 



34. Multiplicity Reactivation. — (Luria, 1947; Luria and Dulbecco, 

 1949) 



