A. D. HERSHEY 



persists in the infected cell for an average of 8 minutes. Labeled 

 viral precursor DNA, under the same conditions, persists about 

 14 minutes. This difference seems to show that the cell con- 

 tains more precursor DNA than precursor protein, and therefore 

 that the DNA destined for a given phage particle is formed some- 

 what earlier than the protein destined for that particle. 



Both the serological and tracer methods are consistent with 

 the idea that protein synthesis is a terminal phase in the forma- 

 tion of the phage particle, but neither has yielded satisfactory 

 proof of this idea. A more promising approach might be to 

 look for conditions under which maturation is inhibited while 

 DNA synthesis proceeds, or conditions permitting DNA syn- 

 thesis but not protein synthesis. Cultures containing proflavine 

 furnish an example of the first class. This substance appears to 

 block only a late step in maturation and permits both protein 

 and DNA synthesis (44). One inhibitor of protein synthesis has 

 been tested. Cohen (7) finds that 5-methyl tryptophan blocks 

 both protein and DNA synthesis in infected bacteria. This 

 suggests either that vegetative phage contains protein or that 

 DNA synthesis is dependent on the synthesis of protein for other 

 reasons. The attempt to confirm or refute this conclusion by 

 studying the effects of specific metabolic blockade seems at 

 present to offer the most general method of defining the com- 

 position of vegetative phage. 



RNA ECONOMY IN INFECTED BACTERIA 



Cohen (8) showed that the amount of RNA in infected 

 bacteria remains constant from the moment of infection, and 

 that not more than 2 to 9 per cent of its phosphorus is replaced by 

 newly assimilated phosphorus during 60 minutes of viral growth. 

 Manson (48) confirmed the latter result by a more rigorous 

 method ; in his experiments replacement up to 5 per cent of the 

 total RNA could not be excluded because of the presence of 

 uninfected bacteria in the cultures. In order to see what these 

 results mean it is necessary to make some quantitative com- 

 parisons. 



14 



