252 BACTERIOPHAGES 



most if not all of the phage protein is derived from materials as- 

 similated from the growth medium after phage infection. 



The kinetics of protein synthesis in phage-infected bacteria has 

 been investigated in several laboratories. Cohen (1947 a, 1948) 

 found that protein synthesis in T2-infected bacteria proceeded 

 without interruption, although the net synthesis of RNA and 

 DNA was stopped. The protein increment was not characterized 

 to determine whether it was of phage or bacterial specificity. 

 Levin thai and Fisher (1952, 1953), by breaking open T2-in- 

 fected bacteria at intervals during the latent period, observed 

 toroid-shaped objects ("doughnuts") which appeared before 

 mature phage, increased in number during early stages of phage 

 maturation, and then decreased toward the end of the latent 

 period. These objects were apparently phage precursors, re- 

 sembling empty phage heads in shape and size and being agglu- 

 tinated by antibodies to phage heads. They did not adsorb to 

 the host cells because the organ for attachment, the tail, was 

 lacking. A small number of doughnuts with tails were also ob- 

 served in these premature lysates. Similarly DeMars, Luria, 

 Fisher, and Levinthal (1953) and DeMars (1955) reported the 

 detection of soluble phage antigens in T2-infected bacteria which 

 were not part of either mature phage particles or "doughnuts." 

 These soluble antigens were characterized by their ability to 

 block phage-neutralizing antibodies. These experiments dem- 

 onstrate that proteins with the specificity of phage but not yet 

 built into mature phage are present in phage-infected bacteria 

 during the latent period. Similar experiments with T5-infected 

 bacteria were reported by Y. T. Lanni (1954). 



A different kind of phage precursor was detected in T2-in- 

 fected bacteria by Maal0e and Symonds (1953) by the use of S^^ 

 from the medium as a label. These precursors contained protein 

 but no nucleic acid, sedimented more slowly than mature phage, 

 adsorbed to sensitive bacteria but did not kill them, and were 

 agglutinated by antiphage serum, lliese properties resemble 

 those of "osmotic ghosts" of phage T2. Their number was ap- 

 proximately constant from 15 minutes after infection until lysis 



