346 R. DULBECCO [voL. 59 



phage complex do not co-operate to produce PHTR but that each quantum in- 

 dividually has a chance to accomplish PHTR and that this chance is independent 

 of any other quanta absorbed by the complex. We would thus be led to conjec- 

 ture that PHTR is due to a primary photochemical reaction. 



(5) The picture is complicated by the finding that at high intensities the rate 

 of PHTR ceases to be proportional to the intensity of illumination, reaching a 

 maximum value, and by the finding of a complex temperature dependence. These 

 findings require the participation of dark reactions in the mechanism of PHTR. 



(6) The probability of PHTR per time unit (PHTR rate) is practically in- 

 dependent of the dose of U\^ used for inactivation. This finding shows that 

 photoreactivation is an all-or-none phenomenon, and it may indicate that photo- 

 reactivable inactivation is always due to one injury, elimination of which resti- 

 tutes activity. 



To explain all the Icnown features of PHTR, we propose the following working 

 hypothesis: The photoreactivable inactivation is due to formation of molecules 

 of an inhibitor in the phage; although many of these molecules may be formed 

 in one phage particle, just one molecule is the inactivating one in each case, 

 for example, by blocking a reaction necessary for phage growth in a small area 

 of the contact surface between phage and bacterium. Restoration of phage ac- 

 tivity requires the permanent removal of the inactivating inhibitor molecule. 

 Dissociation does not occur by thermal activation; but absorption of a light 

 quantum of a given wave length produces a transient and reversible dissociation. 

 During the time the inhibitor is dissociated it can be captured by a receptor and 

 destroyed with dark reaction. This makes the removal permanent and consti- 

 tutes reactivation. 



PHTR therefore requires a system made by phage, inhibitor, pigment, and 

 receptor. The inhibitor belongs to the phage, the receptor to the bacterium being 

 perhaps of enzymatic nature ; the pigment may belong to either one and may be 

 identified either with the inhibitor or with the receptor. The system is com- 

 pleted after adsorption of the inactive phage on bacteria. 



The probability of PHTR (PHTR rate) for low light intensity is proportional 

 to the time integral in which the inhibitor is dissociated and therefore to the 

 number of quanta absorbed (dose of the reactivating light). For high intensities 

 the activation times due to absorption of different quanta overlap somewhat, 

 so that equal doses become less efficient. When the light intensity is so high that 

 the inhibitor is dissociated without interruption during illumination, the proba- 

 bility of PHTR reaches a maximum value. The probability of PHTR is also pro- 

 portional to the probability that the dissociated inhibitor will be captured and 

 destroyed by the receptor in the time unit and therefore depends on temper- 

 ature, which influences the dark reactions, and on some physiological conditions 

 of the bacteria, which may affect the efficiency or the amount of the receptor. 



REFERENCES 



BowEN, E. J. 1946 The chemical aspects of light. Clarendon Press, Oxford. 

 DuLBEcco, R. 1949 Reactivation of ultraviolet-inactivated bacteriophage by visible 

 light. Nature, 163, 949-950. 



245 



