PHOTOKEACTIVATION 461 



one type of damage is produced but not all the bacteriophage particles are 

 exposed to the photoreactivating mechanism. 



The cross section of phage particles to ultraviolet can thus be sub- 

 divided formally into two sectors, only one of which is photoreactivable. 

 The nonphotoreactivable sector is measured by the ratio 



slope of straight part of survival curve after photoreactivation 

 ~ slope of straight part of survival curve in darkness 



The photoreactivable sector a may be taken as a measure of the photo- 

 reactivability of different phages. Data for the seven phages of the T 

 group adsorbed on E. coli B are given in Table 12-1. It is interesting to 



Table 12-1. Photoreactivity of the Phages of the T Group 



note that among the related phages T2, T4, and T6, phage T4, which is 

 much less photoreactivable, apparently contains less nucleic acid than the 

 other two (Luria et al., 1951). A similar correlation occurs also in plant 

 viruses (Sect. 4). 



The ratio of the two sectors of the total cross section is not a charac- 

 teristic of the phage alone, but is somewhat affected by the host bacterium 

 in which photoreactivation occurs. Phage T2 has a slightly higher photo- 

 reactivable sector if adsorbed on E. coli B/1 than if adsorbed on E. coli B 

 (Dulbecco, unpublished). 



3-5. KINETICS OF PHOTOREACTIVATION IN PHAGE T2 IN CONDITION 



OF SINGLE INFECTION 



3-5a. Effect of Different Doses of Photoreactivating Light at Constant 

 Intensity. A sample of ultraviolet-irradiated phage is adsorbed on sensi- 

 tive bacteria in darkness and then exposed to a photoreactivating light of 

 constant intensity; at regular time intervals samples are taken and 

 assayed for active phage. The number of active particles, p{t), increases 

 with the time of illumination; if exposure is continued for a long time a 

 maximum number of active particles, p(=c) is reached, after which any 

 further illumination is without effect. In experiments made with the 

 same ultraviolet-irradiated samples with reactivating light of different 

 intensities, approximately the same maximum is reached, as shown in 



