EFFECT OF RADIATIONS ON PHAGE PARTICLES 71 



reversal of radiation lesions in a number of different organisms. 

 Photoreactivation of bacteriophages was discovered independ- 

 ently by Dulbecco (1950), who observed that ultraviolet- 

 inactivated phages can be reactivated by exposure to visible 

 light, but only after their adsorption to susceptible bacteria. 

 Visible light does not reactivate unadsorbed ultraviolet-ir- 

 radiated phages, nor induce phage development in bacteria il- 

 luminated just before infection with such phages. The rate of 

 photoreactivation of adsorbed phage particles is a function of the 

 temperature, increasing by a factor of two in the interval from 

 25 to 37 ° C. These observations suggested that photoreacti- 

 vation depends on some enzyme system present in the bacterial 

 host cell but absent from the phage particles. Attempts to 

 obtain photoreactivation of free phages in cell-free extracts of 

 bacteria have thus far been unsuccessful, although photoreacti- 

 vation of ultraviolet-inactivated transforming DNA has recently 

 been demonstrated in extracts of E. coli (Goodgal, Rupert, and 

 Herriott, 1957). Photoreactivation of adsorbed phages is not 

 inhibited by cyanide or anaerobiosis. The rate of photoreacti- 

 vation is a linear function of the intensity of the reactivating light 

 at low intensities and approaches a maximum value at high in- 

 tensities, at which point some factor other than absorption of 

 light quanta becomes rate limiting. Light of wavelength near 

 3,650 A possesses the maximum efficiency of photoreactivation, 

 although the entire wavelength range from 3,000 to 5,000 A is 

 effective. If ultraviolet-inactivated phages are adsorbed to 

 rapidly growing bacteria and the infected cells are incubated for 

 various lengths of time before exposure to photoreactivating light, 

 it is found that the maximum amount of photoreactivation at- 

 tainable decreases rapidly. If such phages are adsorbed to 

 resting bacteria, however, the complexes remain photoreactivable 

 at 37 ° C. for at least 70 minutes. The rate of photoreactivation 

 at a given light intensity is less in resting than in rapidly me- 

 tabolizing bacteria. 



Only a portion of ultraviolet-inactivated phage particles is 

 photoreactivable. The fraction of the phage population capable 



