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R. DULBECCO 



[vol. 59 



UV light in the phage has a probability, a, of producing a photoreactivable in- 

 activation and a probability, b, of producing a nonphotoreactivable inactivation; 

 a + 6, the probability of producing any inactivating damage, is proportional 

 to the cross section of the phage for UV. Assuming a -{- b = 1, a is the photo- 

 reactivable sector of the cross section, b the nonphotoreactivable sector; h is 

 measured by the ratio of the slope of the curve after maximum PHTR to the 

 slope of the curve in darkness, both measured in the straight parts. 



The photoreactivable sector, a, varies between 1 (complete photoreactivability) 

 and (no photoreactivability) and can therefore be used as an index of the 

 photoreactivability. Values of a for different phages are given in table 2. 



Influence on PHTR of the Interval of Time between Infection and Exposure to Light 



In the experiments reported in the present and following sections the influence 

 of various experimental conditions on PHTR was analyzed. A quantitative de- 

 termination of PHTR was made by measuring either the "active fraction" or 

 the "amount of PHTR" in an UVP sample after a given exposure to light. The 

 active fraction is the ratio of the number of active particles after PHTR to the 

 total number of adsorbed particles and is equal to the sum of the fraction active 



TABLE 2 

 Photoreactivability of the phages of the T group 



in the darkness plus the fraction reactivated by light; the amount of PHTR is 

 the reactivated fraction. 



The influence on PHTR of the interval of time between infection and exposure 

 to light was determined for UVP adsorbed on bacteria in broth and on resting 

 bacteria (see "Material and Methods"). Bacteria and UVP were mixed in dark- 

 ness; samples of the mixture were kept in darkness for various intervals of time 

 and then exposed to light for a period long enough to produce maximum PHTR. 

 After illumination, samples were plated and incubated in darkness, and the ac- 

 tive fraction was determined. In this procedure the bacteria infected with ir- 

 radiated phage particles had to be exposed to light much longer than the latent 

 period between infection and liberation of phage adsorbed on bacteria in broth. 

 When bacteria in broth were used, therefore, the mixtures were plated before the 

 end of the latent period and illumination was continued by exposing the plates; 

 when resting bacteria were used, illumination could be continued indefinitely in 

 liquid, since no phage liberation takes place under these conditions. 



Experiments with bacteria in broth. The experiments were performed with phage 

 T2 at 28 C. The amount of PHTR decreased rapidly as the time interval between 

 infection and the beginning of exposure to light increased; after about 20 minutes 

 only a small amount of PHTR was produced, as is shown in table 3. 



233 



