PHOTOREACTIVATION 473 



tion, is completely reactivated by the light; the second is similar to that 

 observed in resting cells, and shows the constant ultraviolet dose reduc- 

 tion by light. 



Novick and Szilard interpret the constant dose-reduction phenomenon 

 in the following way. They assume that inactivation by ultraviolet is 

 due to formation of a poisonous chemical compound in the cells, produced 

 in amount proportional to the ultraviolet dose. The poison is produced 

 in two forms; one sensitive, the other insensitive to light, the ratio 

 between the amounts of the two forms in individual bacteria being inde- 

 pendent of the ultraviolet dose. The fraction of survivors after a given 

 ultraviolet dose is determined by the amount of poison of both types 

 present in the cells at the time they are incubated with nutrient medium 

 and permitted to multiply. If the bacterium is exposed to light before 

 this moment, the photosensitive poison is destroyed, and only the photo- 

 insensitive one remains. The formal reduction of the ultraviolet dose is 

 due to reduction in the amount of poison. 



This theory can explain well the observed relations of the survival 

 curves in E. coli B/r and many other features of photoreactivation in this 

 organism (see Sect. 5-4). 



In those microorganisms, in which the survival curve approaches the 

 one-hit type, as in bacteriophages, the amount of the poison able to pro- 

 duce inactivation in the majority of the organisms reduces to a small 

 number of molecules per organism, so that the poison theory of inacti- 

 vation becomes a "hit" theory. 



5-4. KINETICS 



The kinetics of photoreactivation of E. coli B/r has been worked out by 

 Novick and Szilard (1949) as a development of the poison theory. It is 

 assumed that the photosensitive poison is destroyed by the light in a first- 

 order reaction. As a result, the curve of log [1 — p(t)/p{ oo )] versus time 

 (see Sect. 3-5) has a multiple-hit character; this theory could not there- 

 fore explain a type of photoreactivation like that found in phage T2, in 

 which this curve is a straight line. In E. coli B/r the expectation of the 

 theory is fulfilled. The action of the light in the first minutes of illumina- 

 tion is somewhat less than expected on the basis of the theory, so that a 

 latent period of several minutes is assumed. 



The rate of destruction of the poison, calculated according to the 

 theory, is independent of the ultraviolet dose. The calculated rate 

 depends on the intensity of the reactivating light and increases with its 

 intensity but without strict proportionality. The latent period increases 

 when the light intensity decreases. 



If photoreactivation is carried out with a dose which does not give 

 maximum photoreactivation, the survival curves still follow the constant 

 dose reduction principle. 



