70 BACTERIOPHAGES 



(1950) to study the effects of phage infection on the chromatinic 

 structures of the bacterial host cells. These authors also ob- 

 served the effects of infection with ultraviolet-inactivated Tl, T2, 

 and T7 phages. It was found that ultraviolet-inactivated 

 phages, like normal unirradiated particles, cause marked al- 

 terations of cell structure. The details of this work will be 

 discussed in a later chapter. Luria and Human also followed 

 the synthesis of material absorbing ultraviolet light at 2,600 A 

 in bacteria infected with normal and with irradiated phages. 

 They found that infection with irradiated Tl or T2 phages causes 

 some increase in optical density at 2,600 A, but at a much slower 

 rate than in uninfected bacteria. It may be concluded, there- 

 fore, that although ultraviolet-inactivated phage particles are 

 unable to reproduce themselves, they can nonetheless cause ex- 

 tensive changes in the infected bacterium, changes which ulti- 

 mately result in the death of the host cell. 



d. Interference 



Luria and Delbriick (1942) found that infection of bacteria 

 with ultraviolet-inactivated T2 phages makes it impossible for 

 unirradiated particles of Tl to multiply in these cells. In this 

 respect, ultraviolet-inactivated T2 phages behave exactly as do 

 active T2 particles. In contrast, ultraviolet-inactivated Tl 

 phages do not interfere with the multiplication of active T2 

 phage. Infection of bacteria with ultraviolet-inactivated phages 

 also results in interference with induced enzyme synthesis under 

 the same conditions under which active phages produce such 

 interference (Luria, 1950). These results demonstrate that 

 ultraviolet-inactivated phage particles still possess some of the 

 physiological properties of active phages. 



e. Photoreactivation 



Kelner (1949) discovered while working with ultraviolet- 

 inactivated conidia of Streptomyces grisens that visible light has the 

 remarkable ability of restoring viability in this material. Since 

 this observation, there have been numerous reports of the photo- 



