REQUIREMENTS FOR PHAGE PRODUCTION 251 



is adequately pure. In all of the previous papers the final phage 

 yield used for analysis was purified only by differential centrifu- 

 gation. Physical and chemical criteria of purity applied to such 

 preparations are virtually useless as tests of radiochemical purity 

 in experiments of the type under consideration. This point was 

 tested in experiments by Hershey, Garen, Fraser, and Hudis 

 (1954) using phage T2 grown in bacteria labeled with preassimi- 

 lated S^\ The radiochemical purity of the centrifugally isolated 

 phage preparations was measured by following the adsorption of 

 the sulfur label to strain B of E. coli and failure of adsorption to 

 B/2. These tests indicated that indeed most of the radioactivity 

 of the final phage yield was due to contaminating bacterial pro- 

 teins and was not built into the phage particles. Only 2 to 3 

 per cent of the total phage protein could have been derived from 

 labeled bacterial protein, the latter contribution amounting to 

 only 0.4 per cent of the total labeled bacterial protein. Even 

 this small amount may have been derived from bacterial gluta- 

 thione rather than from bacterial protein. These experiments 

 demonstrate quite conclusively that the host cell makes a negligi- 

 ble contribution to the proteins of phage T2. This finding that 

 contaminating bacterial protein is responsible for the host cell 

 isotopic label found in the phage yield is sufficient explanation 

 for the observation by Kozloff', Knowdton, Putnam, and Evans 

 (1951) of an inverse relationship between the phage yield and the 

 apparent host contribution to the phage. If the amount of con- 

 taminating bacterial particles per cell present in the phage yield 

 is approximately constant, as indicated by the results of Hershey, 

 Garen, Fraser, and Hudis (1954), it would constitute a smaller 

 fraction of the yield, as the yield of phage particles per cell in- 

 creased. This might also be the explanation for the apparently 

 large host cell contribution made to the small phage T7 as ob- 

 served by Putnam, Miller, Palm, and Evans (1952). One 

 may conclude that at present there is no definitive evidence that 

 aiiy phage materials are derived from host cell proteins although 

 in the case of phage T2 it is possible that 2 to 3 per cent of the 

 phage protein might be derived from this source. Evidently 



