RADIOBIOLOGY OF BACTERIOPHAGE 



357 



framework of notions generated by our consideration of ultraviolet light 

 acting on T4 or T2 will serve as a background against which to view the 

 other agents and other phages. 



II. Ultraviolet Irradiation of T-Even Bacteriophage 

 A. Irradiation of T4: 

 1. Inactivation of Ivfectivity 



It is usual to define a surviving particle as one which can produce a visible 

 plaque under the standard conditions for phage assay (see Adams, 1950). 

 The advantage of this definition is the convenience of the plaque-assay 

 method. (A potential disadvantage may be that in extreme cases a hetero- 

 geneity in size of plaques among the survivors may lead to some "plaques" 

 being overlooked.) 



40 60 80 

 Time (sec) 



Fig. 2. An ultraviolet light survival curve for T2. A plot of the log fraction of 

 phage surviving versus time of irradiation usually shows a slight initial dowTiward 

 concavity for T2 and T4. This figure is reproduced from Benzer et al. (1950) by permis- 

 sion of the California Institute of Technology. 



In practice, a survival curve of T4 may fail to be strictly exponential. 

 The curve may show a slight shoulder, i.e., an initial downward concavity 

 (Fig. 2). The extent of the shoulder has been found to depend on the wave- 

 length of ultraviolet fight employed (Tessman, 1954). No convincing ex- 

 planation for the shoulder has been offered; Doermann and Chase (personal 



