REQUIREMENTS FOR PHAGE PRODUCTION 257 



and function of DNA, nor with much experimental evidence 

 (Kozloff, 1953, Hershey, Garen, Fraser, and Hudis, 1954). 



5. Kinetic Studies of DNA Metabolism 



Several investigators have used kinetic methods to follow the 

 course of nucleic acid metabolism in phage-infected bacteria. 

 Cohen (1947a, 1948) demonstrated that in T2-infected bacteria 

 net RNA synthesis stopped and there appeared to be an interrup- 

 tion in DNA synthesis after which DNA was produced even more 

 rapidly than in uninfected cells. In infected cells DNA synthesis 

 was well ahead of phage maturation but the two curves were 

 parallel as if both processes were controlled by the same rate- 

 limiting reaction (Cohen, 1949). The rates and total amounts 

 of DNA synthesis in bacteria infected with T2r"^ and T2r phages 

 were compared by Cohen and Arbogast (1950b). Infected cells 

 of both types formed DNA at the same linear rate, but synthesis 

 continued 2 to 3 times longer in the lysis-inhibited cells. These 

 findings were confirmed by Stent and Maal0e (1953) who de- 

 termined by use of P^^-labeling that assimilation of phosphorus 

 from the medium into phage proceeded at the same rate in 

 lysis-inhibited cultures as in cultures before lysis inhibition, and 

 hence that the phenomenon of lysis inhibition merely delays the 

 lytic reaction without directly aff"ecting phage synthesis. 



By the use of isotopic labels Weed and Cohen (1951) showed 

 that T6r^ phage harvested after premature lysis at 35 minutes 

 after infection contained a much higher proportion of pyrimidines 

 originating in the host than did phage harvested after spontane- 

 ous lysis at 5 hours. The quantitative data suggested that es- 

 sentially all available host pyrimidines had been built into phage 

 which had matured during the first 35 minutes after infection. 

 Contrary conclusions drawn from kinetic experiments by Kozloflf, 

 Knowlton, Putnam, and Evans (1951) may be attributable to 

 two causes; the sampling times were 3.5 and 24 hours after in- 

 fection, long after all host contributions had been assimilated 

 into mature phage, and also after phage synthesis had ceased in 

 the lysis-inhibited cultures. 



