250 BACTERIOPHAGES 



at any time before or after bacterial infection and similarly one 

 can dilute out the label at any time. This provides a tremendous 

 flexibility to the experiments and permits one to study the dy- 

 namics of phage synthesis. 



Relatively less information is available as to the source of 

 phage proteins because the major effort has been expended on 

 the currently more interesting problem of nucleic acid metabo- 

 lism. Kozloff, Knowlton, Putnam, and Evans (1951) studied the 

 source of phage T6 nitrogen compounds by labeling either the 

 bacterium or the growth medium with N^\ They concluded 

 that from 10 to 20 per cent of the phage protein nitrogen is 

 derived from bacterial substances assimilated before infection 

 and the remainder is obtained from the medium after infection. 

 The contribution to phage amounts to about 2 per cent of the 

 bacterial protein indicating that most of the bacterial protein is 

 unavailable for phage synthesis. 



The experiments were extended by Siddiqi, Kozloff, Putnam, 

 and Evans (1952). Using bacteria labeled with N^^ and with 

 C^*-lysine, they concluded that about 1 5 per cent of the phage T6 

 protein could have been derived from the host cell. The source 

 of this material was host cell proteins rather than acid-soluble 

 metabolites. Only 1 to 2 per cent of the bacterial lysine was 

 available for synthesis of phage protein. Most of the virus pro- 

 tein must be synthesized de novo from materials in the growth 

 medium. 



Putnam, Miller, Palm, and Evans (1952) isolated the protein 

 from phage T7 grown in bacterial cells labeled with N^^ and 

 concluded that about 40 per cent of the phage protein nitrogen 

 was derived from bacterial substances assimilated before infec- 

 tion and the remainder was derived from the medium after in- 

 fection. The increased fraction of host protein in phage T7 as 

 compared with phage T6 is probably a reflection of the smaller 

 size of phage T7, since only a small amount of bacterial sub- 

 stance is utilized in either case. 



There is one major difficulty with all these experiments, and 

 that is the difficulty in determining when the phage preparation 



