170 KARL C. LARK 



synthesis of DNA in vivo in l)oth l)act('iia and animal cells (Piusoff, 

 1959; Morris and Fischer. 19G0; Lark. 19b0; Longer and Klenow, 1960). 

 Longer and Klenow (1960) have shown that whereas deoxyadenosine 

 will inhibit the synthesis of DNA by ascites tumor cells, this inhibition 

 can be overcome by the addition of deoxygiianosine. The inhibition is 

 accompanied by the accumulation, intracellularly, of deoxyadenosine 

 triphosphate (Munsch-Peterson, 1960). Other studies have indicated 

 that this is responsible for the block in DNA synthesis (Overgaard- 

 Hansen and Klenow, 1961; Klenow, 1961). On the other hand, these 

 effects have been demonstrated to date only in the presence of extremely 

 high concentrations of deoxy nucleosides in the medium, whereas purine 

 deoxy riboside triphosphates have not been observed to accumulate in 

 normally growing cells. For these reasons it is not clear whether this 

 method of regulation is used to control the supply of precursors needed 

 for DXA biosynthesis. 



The possibility also exists that the rate-limiting ste]) in the synthesis 

 of DNA precursors may be the availability of ribonucleotides from 

 which deoxyribonucleotidcs are formed. A system of this type has been 

 obtained from phage-infected cells by Cohen et al. (1961), who have 

 studied the in vitro conversion of RNA to deoxyribonucleotides. They 

 have noted the fact that RNA may play a key role in the control of 

 DNA and protein biosynthesis (see Section III below) in that an RNA 

 molecule may serve as substrate in the synthesis of the former or tem- 

 plate in the synthesis of the latter but cannot be both template and 

 substrate at the same time. They point out, however, that such a control 

 mechanism can only operate in systems where ribonucleotides are 

 limited in availability. This does not appear to be the case in bacteria. 

 In synchronized bacteria, in which DNA is being synthesized in steps, 

 large amounts of ribonucleotides are present at all times during the cell 

 cycle, (Maruyama and Lark, 1962) and bacteria, which synthesize DNA 

 and RNA at different rates, adjust tlic intracellular concentration of 

 ribonucleotides to corresiiond to these rates (Franzen and BinkU^y. 1961 ; 

 Smith and ]Maal0e, 1962 1. In all cases, the intracellular ribonucleotide 

 content is ample to provide material for DNA synthesis, and DNA and 

 protein synthesis i)roceed simultaneously. 



B. THE MEASIREMENT OF ENZYME ACTIVITY ASSOCIATED M'lTH THE 

 PRODUCTION OF DNA PRECURSOR 



This indirect approach has been used by several workers interested 

 in the enzymatic basis of the control of DNA synthesis. 



In interpreting data of this typo, it must be remembered that the 



