72 BACTERIOPHAGES 



of forming plaques after maximum photoreactivation decreases 

 as an exponential function of the dose of ultraviolet light, but less 

 rapidly than the fraction which survives in the dark, i. e., in the 

 absence of photoreactivation. If the absorption of ultraviolet 

 light in a given type of phage has a probability a of producing a 

 photoreactivable lethal lesion and probability \-a of producing 

 a nonphotoreactivable lethal lesion, then a is said to be the 

 photoreactivable sector of ultraviolet damage in this phage 

 strain. The ratio of the slope of the dose-survival curve after 

 maximum photoreactivation to the slope of the dose-survival 

 in the dark, or in the absence of photoreactivation, is then 

 evidently 1 -a. The photoreactivable sector a may vary from unity 

 (complete photoreactivability) to zero (no photoreactivability) 

 in different phage strains. Experimental values for the photo- 

 reactivable sectors of the 7 T phages are : Tl = 0.68 ; T2 = 0.56 ; 

 T4 = 0.20; T6 = 0.44; T3 = 0.39; T7 = 0.35; T5 = 0.20. 

 These values show no simple correlation with any of the other 

 properties of the phage concerned ; for instance, the three closely 

 related phages T2, T4, and T6 are seen to differ widely in their 

 photoreactivable sectors. In spite of this, the photoreactivable 

 sector is a remarkably reproducible characteristic of a particular 

 phage strain, although it depends on the physical conditions 

 under which the ultraviolet light has been administered (Hill 

 and Rossi, 1954). The photoreactivable sector also depends 

 somewhat on the type of host cell to which the ultraviolet-in- 

 activated phage particles have been adsorbed (Dulbecco, 1955). 



By means of experiments in which phage-infected bacteria 

 were exposed to flashes of photoreactivating light rather than to 

 continuous illumination it could be shown by Bowen (1953) 

 that photoreactivation consists of at least two steps. One, a 

 temperature-sensitive dark reaction (i. e., one for which no light 

 is required) appears to precede a second, temperature-insensi- 

 tive light reaction (for which light is required). Bowen pro- 

 posed that the function of the dark reaction is to generate those 

 substances which absorb the visible radiations and thus become 



