216 ROBERT L. SINSHEIMER 



Delay in the addition of chloramphenicol to 3 minutes after infection, 

 however, permits the shift of the pattern of RNA synthesis. RNA made 

 under these conditions has the nucleotide composition mimicking that of 

 the phage DNA; no phage DNA is made, however, and the RNA turnover 

 is not observed unless the chloramphenicol is removed. After removal of the 

 chloramphenicol, phage DNA is made and the RNA turnover begins. 



There is evidently a requirement for prior or comcomitant protein syn- 

 thesis during the very early stages of phage infection for the transition 

 of the pattern of RNA synthesis to be achieved. 



If chloramphenicol is added after 9 minutes of infection, DNA synthesis, 

 as previously described, continues; RNA synthesis is apparently accelerated 

 by the addition of the chloramphenicol and RNA turnover continues. 



d. Infection with Ultraviolet Irradiated Bacteriophage 



Ultraviolet irradiation destroys the ability of bacteriophage to reproduce 

 in single infection. If, however, the same irradiated phage preparation is 

 used to infect bacteria at a high multiplicity, so that almost all cells are 

 infected by two or more particles, it is found that a large fraction give rise 

 to progeny (multiplicity reactivation; see p. 192). 



Investigations of DNA synthesis in cells multiply infected with ultra- 

 violet irradiated phage 98, " have shown that the onset of phage DNA 

 production is delayed by some 5 to 10 minutes beyond the time observed 

 in normal infection. Once initiated, however, the rate and amount of DNA 

 synthesis increase to levels comparable to those observed in normal in- 

 fection, but always delayed in time. 



After heavy ultraviolet irradiation there is little multiplicity reactiva- 

 tion and correspondingly there is very little synthesis of phage DNA in the 

 infected culture. 98 



Studies of the rates of synthesis of the enzymes of phage infection under 

 these conditions would be most instructive. Flaks et al. 119 have reported 

 that the synthesis of deoxycy tidy late hydroxy met hylase is observed at a 

 normal rate upon infection with T6r so heavily irradiated that it was 

 incapable of multiplication or of appreciable multiplicity reactivation. 

 Keck et a/. 125 have made similar observations upon the appearance of the 

 dCMP deaminase. 



4. The Relationship between DNA and the Phage Genome 



a. The Size of a Genetic Locus 



(1) Fine Structure Genetics. The results of many experiments of diverse 

 type find a common explanation in the assumption that the DNA of T-even 



might be the result of small differences in the conduct of the experiment if the 

 time of addition of chloramphenicol is critical. 



