REQUIREMENTS FOR PHAGE PRODUCTION 241 



this shift in the synthetic activities of the bacterium following in- 

 fection with T2 phage is sufficient explanation for the long- 

 recognized fact that the net synthesis of bacterial DNA and 

 RNA stops following infection. If this suggestion is correct there 

 still remains the major problem of the mechanism for the shift 

 from cytosine synthesis to the synthesis of HMC. Various 

 possible mechanisms have been discussed in detail by Cohen 

 (1953b), among them the possibility that the infecting phage 

 supplies the essential enzymes for HMC synthesis. 



A possible example of enzymic activity supplied by the infect- 

 ing phage was reported by Earner and Cohen (1954). A mutant 

 strain of E. coli, 15T~, is unable to grow unless thymine or 

 thymidine is supplied. When this strain is infected with phage 

 T2 in the absence of thymine, the phage multiplies and its 

 thymine content ultimately exceeds that of the cells prior to in- 

 fection. The authors suggested two possible explanations, that 

 the infecting virus supplies an enzyme or coenzyme essential for 

 thymine synthesis, or that virus infection releases an inhibition of 

 an enzyme system already present in the uninfected cell. A later 

 paper furnished further information on the metabolism of strain 

 15T~ (Cohen and Earner, 1954). The bacteria were grown in 

 uniformly labeled C^'*-glucose and the nucleic acid bases were iso- 

 lated and analyzed for C^'^ content. The experiments indicate 

 that strain 15T~is able to synthesize thymine at about 4 per cent 

 of the normal rate but much too slowly to permit growth of the 

 bacteria. This finding suggests that phage infection releases an 

 inhibition rather than furnishes a lacking enzyme. These 

 studies demonstrate the difficulty in drawing valid conclusions 

 without a thorough investigation of the system. 



A purine requiring mutant of E. coli strain E which permitted 

 production of phages Tl and T5 in the absence of added purines 

 was reported by Gots in a discussion of the paper of Cohen 

 (1953b). This strain did not permit production of phages T2, 

 T3, and T4 unless a purine was added to the medium. 



Similar experiments with amino acid-requiring strains of bac- 

 teria yielded different results. The infecting phage does not 



