33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 207 



At the same time the bacterial DNA is to a considerable extent degraded 

 and the components used, in part, for the synthesis of phage DNA. 

 (Cytosine of bacterial DNA can serve as a precursor of hydroxymethylcyto- 

 sine of the phage DNA. 102 ) The extent of degradation of the host DNA 

 appears to vary, dependent upon the culture conditions, but as much as 

 two-thirds of the cytosine-containing DNA can disappear from the acid- 

 precipitable fraction in 20 minutes. 63 ' " Phosphorus transfer studies 85 ' 10 ° 

 indicate that in 20 minutes an amount of phosphorus equivalent to 15-20 

 units of phage DNA can be transferred from host to phage DNA. Since 

 in synthetic medium the DNA content of an uninfected cell is only about 

 30 units of phage DNA, this transfer is highly efficient. 



Stent and Maal0e, 103 and Hershey and Melechen 100 have made thorough 

 studies of the origin of the phosphorus found, at various times after in- 

 fection, in mature phage particles and in phage DNA, respectively. The 

 technique has been to expose the cultures to P 32 either well before and 

 during infection, or only until infection, or only during infection, or in 

 brief pulses before or during infection. The results are in generally good 

 agreement. 



A bacterium, in glucose-ammonium medium, at the time of infection 

 contains phosphorus available for conversion to phage DNA phosphorus to 

 the extent of about 50 phage units. Fifteen to 20 units of this are in bacterial 

 DNA, another 25 units are present as transient intermediates, and some 

 possibly are available from bacterial RNA. 100 ' 104 This preassimilated phos- 

 phorus appears, as might be expected, largely in the first mature particles. 

 The first 25 particles per bacterium receive about 50% of the preassimi- 

 lated phosphorus although increasing amounts appear until transfer is 

 complete when there are about 110 particles per bacterium. 



After phage DNA synthesis begins, a pool of phage DNA develops. This 

 pool amounts to some 40 to 80 phage units before the appearance of intact 

 phage particles inside the infected cell (the size of this pool undoubtedly 

 is a function of nutrient conditions) . 



In order to form mature phage particles, the phage DNA must be en- 

 veloped in a protein "coat." Measurements of the incorporation of S 35 , 

 administered at various times to the infected cell, into phage progeny 

 indicate that the synthesis of the precursors of phage protein does not 

 begin until shortly after DNA synthesis begins. Antigenic studies of the 



cells the maximum rate of DNA synthesis during infection is rather less than the 

 combined DNA plus RNA rate before infection. 93 - 10 ° 



102 S. S. Cohen and L. L. Weed, J. Biol. Chem. 209, 789 (1954). 



103 G. S. Stent and O. Maal0e, Biochim. et Biophys. Acta 10, 55 (1953). 



104 A. D. Hershey, A. Garen, D. K. Fraser, and J. D. Hudis, Carnegie Inst. Washing- 

 ton Yearbook 53, 210 (1954). 



