1950] PHOTOKE ACTIVATION OF INACTIVATED BACTERIOPHAGES 339 



a particle is photoreactivated and can be called the PHTR rate; it is proportional 



to the slope of the line giving log f 1 — —, — : j versus time of illumination. Value 



/ may depend on several variables, such as dose of UV, intensity of the reactivat- 

 ing light, temperature, and metabolic condition of the bacteria during PHTR. 

 This dependence will be examined in the next sections. 



Dependence of PHTR rate on dose of UV and intensity of the reactivating light. 

 PHTR rate (/) was determined for UVP inactivated with different UV doses, 

 adsorbed on resting bacteria, and illuminated in liquid at 37 C with light of 

 constant intensity; it was found to be approximately constant for doses of UV 

 between 10 and 30 seconds. The results, however, are not yet definite on this 

 point, and a decrease of / by a factor 1.2 when the UV dose increases from 10 

 to 30 seconds cannot be excluded. This result shows that the probability for an 

 adsorbed quantum to reactivate a photoreactivable phage particle is practically 

 independent of the inactivating UV dose. 



Value / was also determined for different intensities of the reactivating light 

 on UVP inactivated with the same UV dose, adsorbed on resting bacteria, and 

 illuminated in liquid at constant temperature. The intensity was varied either 

 by changing the distance of the sample from an H-4 lamp— assuming the in- 

 tensity to be inversely proportional to the square of the distance — or by lowering 

 the intensity of a monochromatic light by filters and measuring with a thermo- 

 pile the relative intensities. Value / was found to increase almost linearly with 

 light intensity for low intensities but for high intensities to tend to a maximum 

 as is shown in figure 5. The highest value of the PHTR rate observed in these 

 experiments was about 1.4 X 10"^ sec"^ and corresponds to a half-time of about 

 8 minutes. 



For low light intensities, / being a linear function of the intensity, the proba- 

 bility of PHTR occurring in a bacterium-phage complex is a linear function of 

 the dose of the reactivating light (equal to intensity X time), whereas for high 

 intensities the same dose has less effect. For low intensities and relatively short 

 exposures the dependence of amount of PHTR on light dose is also approxi- 

 mately linear. 



Action Spectrum of PHTR 



Seven wave lengths were tested for photoreactivating activity. The corre- 

 sponding monochromatic lights were obtained in the following ways (see Bowen, 

 1946): 



(1) Group of lines near 313 m^t of the mercury arc (with a small amount of 

 334 mju). Light: mercury lamp H-4 without glass envelope. Filter: 3 cm NiSOi- 

 7H2O, 350 g + CoS04-7H20, 10 g made up to a liter with water; 1 cm potas- 

 sium hydrogen phthalate, 5 g in 1,000 ml water. 



(2) Group of lines 365 m^t of the mercury arc. Mercury lamp H-5 (General 

 Electric) ; Coming glass filter combination nos. 738, 5860. 



(3) Group of lines 404 uifi of the mercury arc. Lamp H-5. Filter: 2 

 cm Cu(N03)o-6H20, 200 g in 100 ml water. Iodine 0.75 g in 100 ml carbon tet- 

 rachloride. 



238 



