SYLLABUS 



133 



VII. Reactivation Phenomena 



33. Photoreactivation. — (Dulbecco, 1949b, 1950) Phage inac- 

 tivated by ultraviolet light (UV) (see 11b) is reactivated (i. e., it be- 

 comes again able to give rise to plaques) if exposed to a "photoreactivat- 

 ing" light under suitable conditions. 



The light producing photoreactivation (phtr) belongs to the near 

 ultra-violet and violet part of the spectrum; the action spectrum for 

 phtr shows a band between 3,000 and 5,000 A, with a maximum around 

 3,650 A (Figure 3). Phtr is produced if phage adsorbed on sensitive 

 bacteria is exposed to the photoreactivating Ught: if extracts of bac- 

 teria are substituted for bacteria no phtr occurs. 



When inactive phage particles are adsorbed on washed bacteria 

 resuspended in buffer (resting cells) their photoreactivability lasts for 



WAVE LENGTH 

 Fig. 3. Action spectrum of photoreactivation. The photo- 

 reactivating activity of light of various wave lengths, de- 

 fined as being proportional to the reciprocal of the radiant 

 energy required to produce a standard amount of PHTR, 

 as function of the wave length. 



