IV. CELLrLAR CONTROL OF DNA BIOSYNTHESIS 179 



critical i)rotcin is lost or destroyed, it would have to be of a type other 

 than those involved in the enzymatic synthesis of DNA. Cells in which 

 DNA synthesis has been delayed as a result of irradiation or nitrogen 

 mustard treatment also do not resume this activity if protein synthesis 

 is inhibited (Harold and Ziporin, 1958; Billen, 1959a,b; Harold and 

 Ziporin, 1959; Draculic and Errera, 1959) and, a^ain, either chlor- 

 aniplienicol or starvation of an essential amino acid will pioduce similar 

 effects (Billen, 1961). Extracts of such irradiated cells also do not appear 

 to have lost their enzymatic capacity for in vitro DNA synthesis 

 (Doudney and Billen, 1961). 



Thus, although it appears probable that protein synthesis is normally 

 not necessary to allow a cycle of DNA synthesis once initiated to run 

 to completion, further experimentation is required before it is possible 

 to conclude that protein synthesis must occur in order to initiate a new 

 cycle of synthesis. There can be no doubt that deprivation of amino 

 acids from a cell which requires them will inhibit DNA synthesis. How- 

 ever, it has recently been shown (Gallant and Suskind, 1962; Kellen- 

 berger et ciL, 1962) that this inhibition can be reversed by the addition 

 of chloramphenicol alone. As the period of starvation is increased, how- 

 ever, it becomes more difficult to reinstate DNA synthesis either by 

 addition of chloramphenicol or by readdition of amino acids. This 

 second, irreversible effect may represent a true requirement for protein 

 synthesis. 



Amino acid starvation has also been shown to block RNA synthesis. 

 ]\Ioreover, evidence has accumulated which indicates that only certain 

 types of RNA may accumulate in the presence of low concentrations of 

 chloramphenicol and that such RNA may be unstable (Neidhardt and 

 Gros, 1957; Astrachan and Volkin, 1958; Britten et al, 1962; Nomura 

 and Watson, 1959). Further investigation (Aronson and Spiegelman, 

 1961 a,b) has shown that the amount of RNA synthesized, its stability, 

 and perhaps the type of RNA will depend on the concentration of 

 chloramphenicol used and may depend on the amino acid composition 

 of the medium. Thus, under certain conditions, RNA will accumulate in 

 chloramphenicol-treated cells, whereas in others it may be broken down. 

 Radiation will also inhibit the synthesis of RNA as well as of protein. 

 Thus, in several of the experiments cited above, the controlling factor 

 may be the synthesis of a particular type of RNA rather than protein. 



This is illustrated by several experiments. 



It has been shown that the prolonged synthesis of protein and RNA, 

 under conditions in which DNxA. synthesis is prohibited, results in an 

 increase in the amount of DNA which can be subsequently made in 

 the absence of protein and RNA synthesis. This has been done with a 



