PHOTOREACTIVATION 



469 



tional to the product of the al)sorption coefficient of the photosensitive 

 pigment by the quantum yield. 



The determination of the photoreactivation rate and light intensit}^ 

 yields, therefore, the relative values of the absorption coefficient of the 

 pigment, provided the quantum yield is constant at all wave lengths. On 

 this basis a series of points in an action spectrum for phage T2 in single 

 infection have been obtained, by measuring the intensity reciuired for each 

 wave length to produce a standard low rate of photoreactivation, far from 

 the saturation point (Sect. 3-5b). The light used was reasonably mono- 

 chromatic, having been obtained by isolating lines of the mercury spec- 

 trum by glass and liquid filters. The action spectrum is constituted by a 

 single band, extending from about 3100 up to nearly 4800 A, wdth a maxi- 

 mum between 3600 and 4000 A (Fig. 12-5). The unirradiated phage does 

 not absorb light in this region ; the ultraviolet-irradiated phage has an ab- 

 sorption band between 3100 and 

 3700 A, with a peak at 3300 A. 

 It is very doubtful that light ab- 

 sorption in this band may be re- 

 sponsible for photo-reactivation; 

 more likely the effective light is 

 not absorbed in the phage. 



3-9. ACTION OF CHEMICAL 



SUBSTANCES ON 



PHOTOREACTIVATION 



Reactivation of irradiated bac- 

 teriophages has not been obtained 

 with chemical means, either in the 

 presence or in the absence of light. 

 Extracts obtained from sensitive 

 bacteria cannot replace the bacteria 

 in photoreactivation; samples of 

 ultraviolet-treated phages kept in 

 darkness or in light in the presence 

 of yeast extract, or catalase, are 

 not reactivated. Photoreactiva- 

 tion occurs ec}ually well in the pres- 

 ence and in the absence of oxygen; 

 rate of the process. 



4000 6000 



V»AVE LENGTH, fl 



Fig. 12-5. The action spectrum of photo- 

 reactivation of phage T2. The activity 

 of each wave length, proportional to the 

 reciprocal of the dose of light expressed in 

 ergs per unit area required for obtaining a 

 standard amount of photoreactivation, is 

 plotted versus wave length. The phage 

 was irradiated with the germicidal lamp, 

 adsorbed on bacteria in buffer, and illu- 

 minated in liquid at 37°C. 



0.01 M cyanide does not affect the 



3-10. PHOTOREACTIVATION OF BACTERIOPHAGES INACTIVATED BY 

 AGENTS OTHER THAN ULTRAVIOLET 



A small amount of photoreactivation has been found in phages irradi- 

 ated with X rays (Dulbecco, 1950; Watson, 1950). 



