232 ROBERT L. SINSHEIMER 



to the rather limited extent of available knowledge, a similar biochem- 

 istry of infection. Bacteriophages T3 and T7 are serologically related and 

 and have a similar morphology (Table IV). Tl is serologically different 

 and has a distinctive tail. Because of their common properties it has been 

 suggested 6 • m that they be considered-as members of a common viral class. 



The DNA of phage T7 has a molecular weight of about 14 or 15 X 

 10 6 (Meselson, unpublished data) by both density gradient analysis 67 and 

 calculation from sedimentation velocity. 58 This result suggests the presence 

 of three DNA molecules in each phage particle. 



Net DNA synthesis during the process of infection with these phages 

 is small. Putnam et al. 177 have shown with T7 that 60-90 % of the phos- 

 phorus and the nitrogen of the DNA of the viral progeny is derived from 

 bacterial nucleic acids formed before infection. A smaller proportion (40 %) 

 of the nitrogen of the phage protein is derived from preassimilated bac- 

 terial nitrogen. 



If heavily P 32 -labeled Tl or T7 phages are used for infection, a pro- 

 gressive stabilization of the complex to P 32 decay is observed in the early 

 stages of the phage development just as with the T-even phages. 8a 



According to Crawford, 178, 179 the small increase in DNA content during 

 infection with Tl, T3, or T7 is not blocked by the presence of chloram- 

 phenicol, even when administered shortly before infection. However, Stent 8 " 

 believes that the increase in DNA observed during Tl infection in the 

 presence of chloramphenicol to be a residual increment of host DNA as 

 under these circumstances there is no stabilization of the complex to de- 

 cay of P 32 incorporated into the parental DNA until the chloramphenicol 

 is removed. 



Crawford has also suggested that, unlike the T-even, T5 group of phages, 

 Tl, T3, and T7 do not appear to confer upon the phage-bacterium com- 

 plexes the ability to synthesize thymine. 175 However, the only evidence 

 for this suggestion is that the small increase in DNA during infection 

 with these latter phages is not observed if thymine-requiring bacteria are 

 employed and thymine is not supplied. In view of the small amount (about 

 10 %) of the DNA increase under normal conditions, this observation can- 

 not be regarded as conclusive. 



Amos and Magasanik 179a have shown that uridine in the culture medium can serve 

 as a major precursor of the thymidine of Tl without cleavage of the glycosidic link. 



Crawford 173 has suggested that there is some continuation of net RNA synthesis 

 after infection with the Tl, T3, T7 group. No data of this nature has, however, been 



177 F. W. Putnam, D. Miller, L. Palm, and E. A. Evans, Jr., /. Biol. Chem. 199, 177 

 (1952). 



178 L. V. Crawford, Biochem. J. 65, 178 (1957). 



179 L. V. Crawford, Biochim. et Biophys. Acta 28, 208 (1958). 

 179 * H. Amos and B. Magasanik, J. Biol. Chem. 229, 653 (1957). 



