1950] PHOTOREACTIVATION OF INACTIVATED BACTERIOPHAGES 345 



The behavior of the Qio is similar to that found for complex bacterial activities 

 and for enzymatic reactions (Rahn, 1932) and is an indication that the physio- 

 logical state of the bacterium conditions the probability of the photoreactivating 

 event. 



PHTR of Phage Inactivated with X-Rays 



It was previously reported (1949) that no PHTR had been detected for phage 

 inactivated by X-rays. This statement is valid only if X-ray irradiation is per- 

 formed on phage in synthetic medium. With phage T2 inactivated by X-rays in 

 nutrient broth a slight amount of PHTR can be observed. This amount is proba- 

 bly reduced by the poor adsorption of p^age inactivated by X-rays, as discovered 

 by Watson (1948). After correction for the limited adsorption (data kindly sup- 

 plied by Watson), the PHTR of X-ray -inactivated phage is still considerably 

 less than that for phage inactivated to the same extent by UV. 



SUMMARY AND DISCUSSION 



In the following brief discussion we shall try to arrange the results of our ex- 

 periments in an order that will bring out their theoretical implications, and we 

 shall present a working hypothesis for the mechanism of PHTR. 



(1) The damage caused by UV in bacteriophage consists of two kinds: photo- 

 reactivable and nonphotoreactivable damage. These two kinds of damage occur 

 with comparable cross sections; they may reflect the presence of two kinds of 

 UV-absorbing constituents in each phage particle. Further information should 

 be obtainable by determining the action spectrum for the two types of inac- 

 tivation. 



(2) Only phage particles adsorbed on bacteria undergo PHTR; PHTR can 

 occur within a few seconds after adsorption, indicating that PHTR is due to 

 reactions occurring in the early phase of the interaction between inactive phage 

 and bacterium; perhaps surface reactions are involved, and this may account 

 for the failure to reproduce PHTR with bacterial extracts. PHTR does not re- 

 quire the presence of external metabolic substrates or of oxygen and is not in- 

 hibited by cyanide, but is influenced by the physiological condition of the bac- 

 teria after infection. 



(3) The photosensitive pigment has an action spectrum with a maximum near 

 365 m/x. Normal phage does not have an absorption band corresponding to this 

 action spectrum. UV-treated phage shows an absorption band with a maximum 

 near 330 m/z. Perhaps this is the photosensitive pigment created by UV irradia- 

 tion in the phage. The shift from 330 to 365 m/x could be due to the binding of 

 the pigment with bacterial constituents upon adsorption of a phage particle on 

 the bacterium. The alternative possibility is that the photosensitive pigment 

 exists in the bacterium prior to infection. Further studies of the absorption band 

 of UV-irradiated phage and of the action spectrum of PHTR are needed. 



(4) At low intensities of illumination the probability of PHTR occurring in a 

 bacterium-phage complex is proportional to the dose of light. From this we 

 might conclude that the individual light quanta absorbed by the bacterium- 



244 



