408 RADIATION HIOLOr.V 



violet aiul is tlKM'cforo rosponsihle for the phenomena of multij)hcity reac- 

 tivation. Tlie pli()t()re;i('ti\'ation curve of T3 has amultiple-hitcharacter, 

 with the iHimher of hits proportional to the ultraviolet dose. The mimher 

 of activ(^ particles .S aft(M- a tim(> i follows strictly the e(juation 



S = Be 



ne-f 



> 



where B is the total number of photoreactivahle particles, /; the average 

 number of photoreactivahle hits per phage particle, and / the photo- 

 reactivation rate. I'nder the assumption that both inactivating and 

 photoreactivating events are distributed at random in the phage popula- 

 tion, this relation shows that a photoreactivahle particle of T3 recjuires, 

 to be reactivated, the occurrence of a number of efTective events equal to 

 the number of inactivating events. 



3-7. KINETICS OF PHOTOREACTIVATION OF PHAGE T2 

 IN CONDITION OF MULTIPLE INFECTION 



A bacterium infected with more than one particle of phage T2 inacti- 

 vated with ultraviolet has a much higher probability of yielding active 

 phage than if infected with one particle only (Luria, 1947; Luria and 

 Dulbeeco, 1949). The photoreactivation curve for such multiple- 

 infected bacteria (multicomplexes) is similar to that observed in T3, but 

 does not follow strictly the equation realized in T3, although it tends to it 

 for increasing multiplicity and increasing ultraviolet dose (Dulbeeco. 

 unpublished). These findings are taken as indication that phage T2 upon 

 ultraviolet irradiation receives damages in two different structures, of 

 which one, more sensitive to ultraviolet, is photoreactivated by one photo- 

 reactivating event ; the other, less sensitive to ultraviolet, is photoreacti- 

 vated like T3. In single infection the damage produced in the more 

 sensitive structure is predominating; in multiple infection the phage can 

 partly dispen.se with the sensitive part, the more completely so the higher 

 the multiplicity, so that the ultraviolet damage in the other structure 

 becomes predominant. 



3-8. ACTION SPECTRUM OF THE PHOTOREACTIVATING LIGHT 



Bacteriophage T2 adsorbed on E. coli B is an excellent material for the 

 determination of the action spectrum of the reactivating light, because the 

 rate of photoreactivation, experimentally determinable, is proportional to 

 the concentration of the immediate photoproducts of N* [Sect. 3-5d. 

 Eq. (12-3)]. If the intensity of the photoreactivating light is sufficiently 

 low, we can write: Photoreactivation rate = k-ilC, where C = ^'li^I] 

 (A'4 + A- 5), a constant at constant temperature for a given bacterial 

 population. 



If the light intensity is measured in quanta per unit time, A-3 is proper- 



