518 BACTERIOPHAGES 



particles contain about 40 per cent nucleic acid in contrast to 

 20 per cent for the host cell, and since apparently only phage 

 nucleic acid is synthesized following infection, it should be pos- 

 sible to study phage nucleic acid synthesis in infected bacteria by 

 measuring changes in light absorption at 2600 A in the Beck- 

 man spectrophotometer. In cultures of E. coli growing in a 

 chemically defined medium the absorption at 2600 A increases 

 exponentially with time, doubling in 1 generation time. In 

 cultures of E. coli infected with a 5-fold multiplicity of phage 

 T2, nucleic acid synthesis as measured by absorption at 2600 A 

 ceases completely for 5-10 min. following infection, then pro- 

 ceeds at a rate that is a linear function of time. . . . The concen- 

 tration of bacteria should be about 10^/ml. and the medium 

 and reagents used must not absorb too much light at 2600 A. 



Radiation Alethods 



As discussed in the preceding section, the inactivation of free 

 phage by ultraviolet light is a simple exponential function of the 

 dose of radiation as might be expected if 1 hit were enough to 

 inactivate a phage particle. It is possible that individual 

 phage particles inside an infected bacterium might be inac- 

 tivated in the same way and that the destruction of the plaque- 

 forming potentiality of an infected bacterium would require as 

 many hits as the number of phage particles it contained at the 

 time of irradiation. The destruction of infective centers at a 

 given time during the latent period would follow the course of a 

 multiple hit curve if 2 or more phage particles were present, 

 and a series of such curves at appropriate intervals during the 

 latent period would enable one to determine the course of virus 

 synthesis. 



This type of analysis was attempted by Luria and Latarjet 

 (1947). The bacteria were singly infected with T2 phage, 

 diluted into anti-T2 serum to inactivate free phage and further 

 diluted in a chemically defined medium of low ultraviolet ab- 

 sorption. Samples of the highly diluted suspension of infectiv^e 

 centers were taken at intervals, exposed to ultraviolet irradiation 



