REQUIREMENTS FOR PHAGE PRODUCTION 259 



contradiction to the results of Weed and Cohen (1951), of Stent 

 and Maal0c (1953), and of Hershey to be discussed next. These 

 authors all agree that the host contribution is greater to the 

 phage particles that mature earlier. 



The kinetic work has led to the concept of a series of pools of 

 raw materials at different levels of organization which contribute 

 to the substance of the mature phage. For instance, there is a 

 pool of transient intermediates that is fed from the medium and 

 perhaps from the host cell RNA and DNA, and from which 

 phosphorus is withdrawn for the synthesis of phage DNA. There 

 may be, in fact, a pool for each nucleoside and nucleotide in- 

 volved in phage synthesis, similarly nourished from various 

 sources. Lastly there is the pool of specific phage DNA, nour- 

 ished from all the other pools and the only one to be drawn upon 

 directly by maturing phage particles. Of these the most interest- 

 ing is the pool of phage-precursor DNA (Hershey, 1953a). 



The infected bacterium contains three operationally dis- 

 tinguishable kinds of DNA: (7) mature phage DNA character- 

 ized by its content of hydroxymethylcytosine (HMC) and by its 

 organization into sedimentable particles with the physiological 

 properties of infectivity, adsorbability to host cells, and pre- 

 cipitability by antiphage serum ; (2) phage precursor DNA, after 

 cell lysis nonsedimentable, precipitable by trichloroacetic acid, 

 sensitive to deoxyribonuclease, and containing HMC; and (3) 

 bacterial DNA with properties similar to phage-precursor DNA 

 except that it contains cytosine in place of HMC. The absolute 

 amounts of these forms of DNA were determined by Hershey, 

 Dixon, and Chase (1953) at various times after infection with 

 phage T2. The amounts were reported for convenience in 

 terms of units of DNA per bacterium (the unit being the amount 

 of DNA contained in one phage particle), although they might 

 have been measured in terms of phosphorus, diphenylamine 

 color, optical density at 260 m/x, cytosine, or HMC. 



In a typical experiment mature phage particles first appear 

 about 10 minutes after infection and increase at a linear rate of 

 about 6 phage per minute per bacterium reaching about 600 



