180 KAKl, (i. LAHK 



mutant of E. coll B, T" liist^, rc(iuiriiig thyniiiK' ami histidine, starved 

 of thymine in the presence of histidine, and subsequently starved of 

 histidine in the presence of thymine. From the data it appears that the 

 amount of new DNA synthesized appears to be proportional to the net 

 amount of RNA which can be synthesized. Chloram])hcnicol could be 

 shown to block this increased capacity for DNA .synthesis (Nakada, 

 1960).^ 



Bacteria starved of an amino acid (tryi>toj)lian I arc unable to synthe- 

 size DNA in the presence of chloramphenicol and tryptophan. Upon 

 preincubation with amino acid, starved cells recover the ability to 

 syntlicsizc DNA in the presence of the drug. The rate at which this 

 ability is recovered has been correlated with the synthesis of RNA 

 fDoudney, 1961b). A similar result was obtained if DNA synthesis was 

 blocked as a result of UV irradiation rather than amino acid starvation 

 (Doudney, 1961a). In both of these experiments some correlation could, 

 of course, be made with the recovery of protein synthesis which is 

 known to depend on RNA synthesis. The frequency of ultraviolet 

 induced mutations (sliown to be maximal if DNA synthesis is allowed 

 to proceed as rapidly as possible) also appears to be greatest if the 

 synthesis of RNA is not inhibited (Doudney and Haas, 1961). 



Finally, it has been demonstrated that the genetic inability to syn- 

 thesize RNA in the absence of amino acids (Stent and Brenner, 1961) 

 also extends to the synthesis of DNA (Kellcnberger et al., 1962). 



Although these findings imply that RNA rather than protein synthe- 

 sis may be necessary for the initiation of DNA synthesis, the results of 

 Okazaki and Okazaki (1959) argue that this is not the case, since they 

 obtained DNA synthesis without RNA synthesis when lactobacilli were 

 grown in the absence of uracil. It has been suggested (Kellenbergcr et al., 

 1962) that both RNA and DNA synthesis are subject to the control 

 mechanism involving amino acid participation. 



IV. The Physical State of DNA as a Factor 

 Controlling Its Replication 



The requirement for pre-existent DNA to act both as a template and 

 a primer in the synthesis of DNA has been well established by m ?'?Y/-o 

 studies (see Chapter I). The availability of intracellular DNA to act in 

 this capacity may play an important role in controlling its own biosyn- 

 thesis. This has been suggested by the following experiments: 



^ Allhough the investigator noted a correlation between the amount of protein 

 synthesized and the amount of DNA which can be made, this appears to be errone- 

 ous, since the comparison is made between net contents of DNA rather than newly 

 synthesized DNA. From the data, the correlation with newly synthesized RN.A 

 appears to be more significant. 



