142 NITROGEN METABOLISM 



for this diversion Cohen has found that infection with phage 

 causes a marked reduction in the abiUty of the cell to form 

 ribose-5-phosphate from glucose-6-phosphate (p. 140), and 

 this presumably may explain why such cells are unable to 

 synthesize RNA. The synthesis of deoxyribose- 5 -phosphate 

 from glucose-6-phosphate is unimpaired and consequently 

 deoxypentose-nucleotide synthesis is unaffected. 



Infection with phage has no visible effect on protein 

 synthesis in the host cell, and although the amount of pro- 

 tein synthesized is in excess of that found in the number of 

 virus particles eventually liberated, it is not yet known 

 whether all the excess is viral protein. The origin of the 

 various substances present in the phage has been determined 

 by using cells whose cellular constituents have been enriched 

 with isotopes and also by suspending the infected cells in 

 media containing the appropriate compounds labelled with 

 isotopes. As far as T2 is concerned, no more than 25% of 

 the phosphorus, protein nitrogen, pyrimidines and purines 

 in the liberated phage is derived from substances present in 

 the host cell at the time of infection. How the virus particle 

 gains control of the anabolic activities of the host cell is 

 still unknown. It is however clear that the term 'self- 

 duplicating particle' should not be used indiscriminately 

 since, for example, the synthesis of protein and nucleic acid 

 in infected cells proceeds at a linear rate, whereas if the 

 system were self-duplicating, one might expect it to be auto- 

 catal5rtic. Moreover, there is evidence that the virus is 

 radically changed during invasion of the host cell, since 

 much of the phosphorus and some of the protein nitrogen 

 of the infecting phage particle soon appears in the medium, 

 and not for an appreciable time after invasion is it possible 

 to isolate infective virus from the host cell. The assembling 

 of the various units into completed virus appears to occur 

 only a short time prior to liberation. In the T2 system, the 

 first recognizable structures to be found in the infected cells 

 are the collapsed heads of the phage containing only a little 

 DPNA, the rest of the latter and the tails being added later. 



