REQUIREMENTS FOR PHAGE PRODUCTION 263 



gesting that certain enzymes may be peculiar to the infected cell 

 but such enzymes are yet to be deinonstrated. 



The materials for phage synthesis are derived from three 

 sources, the infecting phage particle, host cell substance as- 

 similated before infection, and the growth medium after infec- 

 tion. The contribution from parental phage is quantitatively 

 small, because of the large factor of increase in each growth cycle. 

 About 50 per cent of the parental DNA appears in phage 

 progeny. Less than 3 per cent of the parental protein can be 

 transferred to phage progeny. 



The host cell contribution of preassimilated material is quan- 

 titatively important. Probably little or no host cell protein is 

 used in synthesis but essentially all the host cell DNA is con- 

 verted into phage DNA. In the case of phage T2, 20 to 30 per 

 cent of the progeny DNA is derived from host cell DNA, a small 

 amount from host cell RNA, and the remainder from transient 

 intermediates and from the growth medium after infection. The 

 phage protein is derived almost exclusively from materials as- 

 similated from the growth medium after infection. 



Kinetic studies reveal that specific phage proteins appear in in- 

 fected cells before mature phage particles, and that these phage 

 proteins are probably precursors of mature phage. The phage 

 proteins are found in three forms, soluble protein with the anti- 

 genic specificity of phage tails, proteins organized as empty phage 

 heads, and perhaps empty phage heads with tails attached. Ap- 

 parently the only organized structures which contain DNA are 

 the fully mature phage particles. It takes about 2 minutes on 

 the average for sulfate from the medium to be assimilated into 

 protein and about 6 minutes more for this protein to be con- 

 verted into mature phage particles. A large amount of unidenti- 

 fied protein is synthesized in phage-infected bacteria immediately 

 after infection. The function of this protein is unknown. 



Kinetic studies indicate that phage DNA synthesis starts soon 

 after infection and at the same time bacterial DNA is broken 

 down and resynthesized into phage DNA. Phage DNA is pro- 

 duced at a rate suflficient for 6 to 8 phage particles per minute per 



