68 BACTERIOPHAGES 



lambda is inactivated by ultraviolet light at only V13 the rate of 

 phage T5, although these two phages have almost the same size 

 (Weigle and Delbriick, 1951). As we will see later, there is 

 much evidence that different strains of phages react very dif- 

 ferently towards ultraviolet light. 



Latarjet and Morenne (1951) found that phage T2 is in- 

 activated slowly when irradiated by Mazda fluorescent lamps of 

 the daylight type. Examination of the radiations emitted by 

 these lamps showed that appreciable amounts of radiant energy 

 emanate at wavelengths from 3,130 A down to 2,890 A, and 

 calculations indicate that these radiations can account for the 

 observed rate of phage inactivation. The kinetics of inacti- 

 vation by fluorescent lamps are not strictly exponential but 

 follow a "3-hit" curve, which suggests that the mechanism of 

 ultraviolet inactivation of these phages may be more complex 

 than had been imagined previously. A downward concavity 

 of the exponential survival curve has also been noted at low doses 

 of ultraviolet light for phages T2, T4, T5, and T6, but the sig- 

 nificance of these deviations from first-order kinetics is not known. 

 In contrast, the ultraviolet light survival curves of phages Tl, 

 T3, and T7 show an upward concavity at low survival values, as 

 if a fraction of the phage is inactivated at a slower rate than the 

 bulk of the population (Dulbecco, 1950). This break in the 

 survival curve of such phages, as well as the low intrinsic ul- 

 traviolet sensitivity of the salmonella phage P22, led Garen and 

 Zinder (1955) to propose that some of the particles in ultraviolet- 

 irradiated populations of such phage types are reactivated by 

 homologous genetic material present in the nuclear structures of 

 the bacterial host cell. A comprehensive discussion of this 

 hypothesis is given by Stent (1958). 



b. Growth- Delaying Effect 



Luria (1944) noted that the latent period of bacteriophage 

 particles surviving moderate doses of ultraviolet light is con- 

 siderably increased. This observation indicates that phage 

 particles can be damaged by the action of absorbed ultraviolet 



