338 



R. DULBECCO 



[vol. 59 



the reactivating light and for different doses of the inactivating UV. A curve of 

 this type is reproduced in figure 4. The hnearity of the experimental curves was 

 found to be statistically significant by comparing, with the x" test, the experi- 

 mental data for the active fractions with data calculated on this assumption, as 

 is shown in table 5. This result shows that PHTR is a one-hit phenomenon; a 



40 &0 



Time of IHuminafion ^minufes^ 



Figure 4. The logarithm of the fraction of photoreactivable particles that has not been 



reactivated after a given time of illumination ( 1 — -— — r ) plotted against the time of illum- 



\ P(«) / 

 ination (in minutes). Phage T2r was irradiated for 20 seconds with the germicidal lamp, 

 adsorbed on resting bacteria, and illuminated in liquid at 37 C. 



TABLE 5 



A comparison between observed and calculated active fractions after different times of 



illumination 



photoreactivable particle is reactivated by one quantum only, independently of 

 the UV dose. 



The dependence of the amount of PHTR on the time of illumination is ex- 

 pressed by the equation 



p(t) = (1 - e-n F(D) 



in which t is the time of illumination, F{D) the photoreactivable fraction, which 

 is a function of the dose, D, of UV. Value / is the probability per time unit that 



237 



