256 BACTERIOPHAGES 



tion of various components is in .general more interesting and has 

 occasionally been measured. The bacterial DNA (labeled in 

 thymine) is utilized almost completely (Kozloff, 1953; Hershey, 

 Garen, Fraser, and Hudis, 1954), and supplies raw materials suf- 

 ficient to make about 30 phage particles per bacterium (Hershey 

 and Melechen, 1957). The bacterial RNA, on the other hand, is 

 utilized very inefficiently (Putnam, 1952). A bacterium that 

 contains RNA equivalent in mass to the DNA content of more 

 than 200 phage particles actually furnishes purines and pyrimi- 

 dines to DNA sufficient to make only about 10 particles (Hershey, 

 Garen, Fraser, and Hudis, 1954). Transient intermediates plus 

 RNA present in the cell at the time of infection furnish phos- 

 phorus to DNA sufficient to make about 30 phage particles 

 (Hershey and Melechen, 1957). Thus the bacterial cell at the 

 time of infection contains several forms of phosphorus that is 

 available for incorporation into phage, the total amount being 

 sufficient to make about 60 particles of phage T2 per bac- 

 terium. The actual amount found in the isolated phage particles 

 may be as high as 50 phage equivalent units per bacterium (Her- 

 shey and Melechen, 1957) or may be much lower, depending on 

 the experimental conditions and techniques. 



The chief point of interest in these facts is concerned with the 

 significance of the efficient conversion of bacterial DNA into 

 phage DNA. At the present time all the known facts are con- 

 sistent with the idea that it serves solely as a source of raw ma- 

 terials. Thus the amount of preassimilated phosphorus per 

 bacterium available for conversion into different phages is about 

 the same, which has the eflfect, puzzling at first sight, of causing 

 the contribution per phage to be about the same for large and 

 small phages (Labaw, 1951 ; Hershey, 1953b), and also has the 

 effect of causing the contribution from the medium after infection 

 to be very small for small phages like T7 (Putnam, Miller, Palm, 

 and Evans, 1 952 ; Labaw, 1 953) . The diff'erences in composition 

 between bacterial DNA and the DNA of T2 call for considerable 

 reorganization. Any direct incorporation of bacterial DNA into 

 phages is not consistent with current ideas about the structure 



