262 G. S. STENT 



being the greater the later protein synthesis has been arrested. Nevertheless 

 no infective progeny particles appear under these conditions until the 

 synthesis of phage precursor protein is allowed to take place by once more 

 removing the inhibitory conditions (Burton, 1955; Melechen, 1955; Tomizawa 

 and Sunakawa, 1956; Hershey and Melechen, 1957). It seems, therefore, that 

 the formation of a nonprecursor protein is required before repHcation of the 

 bacteriophage DNA can commence. Once some of this protem has been made, 

 DNA synthesis can proceed in the absence of further protein synthesis. One 

 might suppose that this protein is an enzyme required for the synthesis of 

 some precursor of the phage DNA, e.g., for HMC, were it not for the observa- 

 tion of Tomizawa and Sunakawa (1956) that the ultraviolet light sensitivity of 

 the T-even infected bacteria does not decrease further after the addition of 

 chloramphenicol, even though more and more phage DNA accumulates 

 within the cell.^ Rather, if the DNA manufactured under these conditions 

 is "good" phage DNA, then it would seem likely that the early, nonpre- 

 cursor protein plays a more profound role in the reproductive process of the 

 vegetative phage than merely being an enzyme of the intermediary metabohsm 

 of the infected ceU. The nature of the phage DNA synthesized in T-even 

 infected bacteria to which chloramphenicol has been added a few minutes 

 after infection has been investigated by Hershey and Melechen (1957). By 

 adding radiophosphorus P^^ to the infected cells only during the time of 

 chloramphenicol inhibition, it could be shown that the DNA synthesized 

 under these conditions does enter the progeny phages which appear after 

 protein synthesis has been once more allowed to proceed. Furthermore, the 

 "normal" DNA formed after removal of the chloramphenicol appears to be 

 diluted in the phage precursor nucleic acid pool constituted by the previously 

 synthesized "chloramphenicol-DNA" before it in its turn finds incorporation 

 into mature progeny particles. The DNA formed in the absence of protein 

 synthesis thus appears to be "good" in the sense of being a genuine phage 

 precursor. 



Not only is it possible for phage DNA to be synthesized in the absence of 

 protein synthesis, but the converse is also true: phage protein can be S}Tithe- 

 sized in the absence of concomitant phage DNA synthesis. This was established 

 by an experiment in which infected bacteria were irradiated with ultraviolet 

 light (TJV) at various stages of the latent period, S^^ label added to the growth 

 medium immediately after irradiation, and the total amount of radioactivity 

 precipitable by antiphage serum, i.e., intracellular antigenic phage protein, 

 estimated after further incubation of the infected culture (Watanabe, 1957). 

 It was observed in this experiment that no phage antigen is ever formed in 



^ This statement refers to the experiment of Lnria and Latarjet (1947), in which the 

 progressive reduction in UV sensitivity of the infective center constitutes an index of 

 the progress of intracellular phage development (cf. Chapter 10, Vol. II). 



