PHOTOUEACTIVATION 



463 



ciency of the first short period of iUumiiiatioii. It can he eliminated by 

 usinji; a fiashinfj; instead of fontinuous li^ht, ;is will be shown in the section 

 on the effect of interrupted liftht (Sect. 3-3(1). A second deviation may 

 occur at hiiih intensities and long exposures, owing to the inactivation 

 phenomenon mentioned previously. These two deviations find an expla- 

 nation in accessory phenomena and do not af!"ect the linearity of the 

 curve in principle. 



The linearity of the curve shows that a photoreactivable particle is 

 reactivated by one hit of the reactivating light, or, in other words, that the 

 transformation of photoreactivable into photoreactivated particles follows 

 a first-order reaction. The slope of the photoreactivation curve is called 



5- 



I 



0.1 



20 



120 



140 



40 60 80 100 



TIME OF ILLUMINATION ,min 



Fig. 12-3. The logarithm of the fraction of photoreactivable particles that has not been 

 reactivated after a given time of illumination, 1 — p(0/p( «= ), plotted against the time 

 of illumination in minutes. Phage T2 was irradiated for 20 sec with the germicidal 

 lamp, adsorbed on bacteria in buffer, and ilhuninated in liquid at 37°C. (Dulhecco, 

 1950.) 



the "rate of photoreactivation." It measures the probability that a 

 given photoreactivable phage particle is photoreactivated in unit time. 

 The fact that T2 phage particles which have received several ultraviolet 

 hits are photoreactivated by one photoreactivating hit is an important 

 item of information. To explain this some particular mechanism either in 

 the inactivation or in the reactivation might be conceived. It might be 

 considered, for example, that ultraviolet inactivation of phages consists in 

 formation of an inhibitor on a given essential nucleotide, and that photo- 

 reactivation consists in the removal of the inhibitor. However, a T2 

 particle contains approximately 5 X 10^ nucleotides, and it adsorbs 10^ 

 ultraviolet quanta for each hit (M. R. Zelle, personal communication). 

 Therefore, for each hit only one-fiftieth of the nucleotides have absorbed a 

 light fiuantum. This absorption would affect the given essential nucleo- 



