186 KARL O. LARK 



ifiu'hi and ( Ji'icluschck (19(32) have i-('iwrtcd tlic dcnaturation of DNA 

 at teniperatures as low as 25° in the presence of certain anions and 

 Keir et al. (1962) have noted that DNA can serve as primer at slightly 

 alkaline pH's (8-9) without having been heated. 



B. IS DNA SYNTHESIS COMPATIHLE WIVU SI.MI LTANKOUS RNA SVNTHKSIS? 



Is DNA, which is s('r\-in<i as a functional tciiiplate for the formation 

 of cellular proteins, simultaneously able to act as a template for its 

 own synthesis? Much recent evidence has accumulated to show that in 

 the synthesis of proteins, RNA serves as a messenger, to carry the 

 necessary information between the DNA template and the ribosome 

 which serves as the site of protein synthesis (Brenner et al., 1961 ; Gros 

 et al, 1961). 



Moreover, it has been shown both in vivo and in vitro that this RNA 

 is formed in combination with DNA and will recomplex with it (Spiegel- 

 man et al., 1961; Geiduschek et al., 1961; Hall and Spiegelman, 1961). 

 In one instance an RNA-DNA-protcin complex thus formed was quite 

 stable (Bonner et al., 1961). 



Under some conditions the ability to replicate DNA may be related 

 to the ability to synthesize messenger RNA. Bacteria which require 

 thymine, arginine, and uracil for growth cease to synthesize DNA when 

 starved of arginine and uracil in the presence of excess thymine. When 

 arginine and uracil are returned to the medium, DNA synthesis resumes 

 gradually after a characteristic lag. This lag in ability to synthesize 

 DNA is independent of the actual synthesis of DNA since it occurs 

 irrespective of whether or not thymine has been omitted from the 

 medium. An identical lag in the ability to induce ^-galactosidase forma- 

 tion is also observed (Roberts, 1960), which is also independent of the 

 actual occurrence of DNA synthesis. (On the other hand, gross protein 

 and RNA synthesis are reinitiated instantly with no apparent lag.) It is 

 possible, therefore, that DNA in such cells is in a physical state which 

 renders a portion of it unabh^ to serve as a template in either RNA or 

 DNA synthesis, oi" alternatively, that its activity is non-s])ecifically re- 

 pressed for a short period of time. 



Experiments on DNA and RNA synthesis in amino acid-starved 

 auxotrophs of E. coli have indicated that synthesis of both can be regu- 

 lated by a common, amino acid-dependent, control mechanism (Kellcn- 

 berger et al., 1962). This mechanism would appc^ar to operate on the 

 ■primer rather than the precursor level. 



No in vitro experiments have been reported, however, in which it 

 was sought to determine whether the synthesis of "messenger" RNA is 

 inhibited by the simultaneous synthesis of DNA or vice versa. Some 



