PHOTOUEACTIVATION 4G7 



After a long dark pciiod, X is piesent at maximum conceiitratioii, 

 iiixcii hy Eq. (12-1); if a \'ery intense light is tui'ncd on, X will l)e very 

 rapidly and almost entirely transformed into X*, which consecjuently 

 will reaeh a high concentration; during further illumination at the same 

 intensity the concentration of N will remain at a lower steady level [given 

 by Eq. (12-2)] and production of N* will continue at a constant rate, 

 lower than the initial one. The photoreactivation curve is, therefore, 

 divided hito two parts: a very short initial part, whose slope is propor- 

 tional to A's/, and therefore not saturated at high light intensity. It 

 measures the velocity of the light reaction; a second longer part, whose 

 slope is proportional to ksl/(k2 + kj). This shows saturation at high 

 light intensities and measures the competition between the dark reaction 

 (12-2) and the light reaction. 



The regeneration of N after a flash of very-high-intensity reactions 

 (12-1) and (12-2) can be studied by exposing the infected bacteria to a 

 series of light flashes of equal length and intensity, separated by dark 

 intervals of various length. According to the model, regeneration of N 

 in darkness should follow the relation 



IN, = MM] (1 - .-.-..) 



where time t is measured from the end of the flash. The experimental 

 data fit this relation very satisfactorily, and determine for reaction (12-2) 

 a time constant of 35 sec at 37°C. Bowen has also studied the effect of 

 temperature on reaction (12-2) by determining /.:2 with three independent 

 methods. The results obtained here are not easily interpretable, because 

 the slope of the Arrhenius plot, as determined with two methods, is not 

 constant ; for temperatures above 20°C the slope corresponds to an activa- 

 tion energy of approximately 9000 cal/mole, and below 20°C to an acti- 

 v^ation energy of approximately 17,000 cal/mole. 



These experiments have therefore proved that both a light and a dark 

 reaction are involved in photoreactivation, have established their 

 sequence, and have measured the time constant of the reverse dark reac- 

 tion from X" to M. 



.3-6. KINETICS OF PHOTOREACTIVATION OF PHAGE T3 



Phage T3 represents the opposite extreme to phage T2 in the T series of 

 phages. It belongs to the smallest group, and has a very short tail. Its 

 radiobiology is in most respects much simpler than that of the larger 

 phages (Dulbecco, unpublished). Its resistance to ultraviolet is about 

 eight times higher than that of free T2, i.e., it is about as high as that 

 found for T2 several minutes after infection (Luria and Latarjet, 1947). 

 Possibly it lacks a special structure which in T2 is more sensitive to ultra- 



