166 THE PHYSICS OF VIRUSES 



covered by Dulbecco (1949, 1950). V^Tien phages which have 

 been inactivated by ultraviolet light (predominantly 2,537 A), 

 so that the plaque count is low, are irradiated ivhile in the bac- 

 terium by very near ultraviolet light, the plaque count rises very 

 definitely. This is the phenomenon of photore activation. Re- 

 turning to the concept that the "enzyme" systems of phage can 

 be damaged or reduced in number by ultraviolet light, there is 

 evidently a condition in the bacterium which is favored by 

 radiation, so that the handicapped phage can find the necessary 

 precursors and survive to the first burst. The action spectrum 

 for photoreactivaticn was measured by Dulbecco. The maximum 

 occurs at 3,700 A and some photoreactivation still occurs at 

 5,000 A, well in the visible. Two classes of inactivated phage 

 exist, one reactivatable, the other not. The fraction reactivata- 

 ble is 0.7 for T-1, which is most reactivatal)le, and averages 

 about 0.4. 



Fluke (1951) has shown that the action spectrum for pro- 

 ducing reactivatable and nonreactivatable phage is substantially 

 the same. The major peak at 2,650 A and the minor peak at 

 2,800 A, as seen in Fig. 6.8, appear in much the same way. 

 Hence, both nucleic-acid and protein absorption play a part in 

 producing the damage, which is subsequently overcome by the 

 photoreactivation. 



Summary and Conclusions from Action Spectra 



We give here a brief summary of the salient features of action 

 spectra and one or two conclusions which can be drawn from 

 them. 



An electronic-vibration absorption is necessary. 



Absorption in nucleic-acid bases or aromatic side chains is 

 most efiicient. 



Polypeptide absorption is very inefficient. 



The quantum yield is low. The number of photons needed is 

 roughly proportional to the surface exposed. 



The quantum-energy threshold is roughly three times the 

 energy of the free-energy barrier for thermal inactivation. 



Inactivation efficiencies are not very temperature dependent. 



