4()4 R \i)i \'n()\ liioi.ocY 



tide only if oiuM-f2;y can be transmitted to it from a group of 50 neighboring 

 nucleotides or if an inhibitor localized on any nucleotide of this group can 

 alTect the essential one, for example, steiically, as well as pre\'ent other 

 inhibitors from localizing on the same group. Both these assumptions 

 seem rather artificial. 



Another explanation of the one-hit character of the T2 photoreacti- 

 vation is to assume that photoreactivable inactivation consists of a 

 luimber of independent damages, at least one per photoreactivable hit, 

 and that one quantum of the reactivating light is able to affect them all, 

 by some kind of trigger mechanism producing numerous molecules 

 capable of photoreactivating. This interpretation is also very artificial. 



Owing to the difficulty in interpreting T2 photoreactivation as a 

 reversal of ultraviolet damage, a different position may be taken, and it 

 may be considered that photoreactivation does not affect the ultraviolet 

 damage itself, but it avoids its consecjuences by making available a sub- 

 stance normally produced in the infected bacterium, which is no longer 

 produced as the consequence of ultraviolet inactivation. As a model, the 

 possibility might be considered that a phage enzyme, E, acting upon a 

 bacterial substrate A, transforms it into B, this step being essential in 

 phage growth, and that the irradiated enzyme is not able to do so any 

 longer. If now the system is exposed to the photoreactivating light, a 

 photochemical product, B*, is produced, which then produces B, and the 

 enzymatic reaction is bypassed. In condition of continuous illumination, 

 in which B* is present in steady concentrations, the probability of pro- 

 ducing B is proportional to the time of illumination, thus giving rise to the 

 one-hit character of photoreactivation. 



A final remark seems to be pertinent at this point. The one-hit char- 

 acter of the photoreactivation curves of this bacteriophage is derived from 

 data extending to approximately 80 per cent of the maximum photoreacti- 

 vation; in this range a one-hit curve could be produced by an inhomo- 

 geneous population composed of different classes in which the number of 

 hits reciuired varies from one to about ten, if the frequencies of the various 

 classes are properly distributed; the proper distribution can be easily 

 calculated. 



3-5b. Effect of Different Light Intensities. Increasing the intensity of 

 the photoreactivating light has the consequence of increasing propor- 

 tionately the photoreactivation rate when the intensity is low; at higher 

 intensities there is less increase in rate, and for very high intensities the 

 rate tends to a maximum value (saturation). A rate-intensity plot gives 

 a hyperbolic curve (Fig. 12-4), which follows the equation 



h + cl 

 Such a dependence shows that the rate of photoreactivation is determined 



