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KARL G. LARK 



lliyiuidiiic (lipliospliatr kiiiasi's lia.s slunvii that these enzynies a])i)eai- 

 sequentially in that order during regeneration (Weissman et al., 1960). 

 Similar results were obtained by the same workers studying enzyme 

 production by L cells of mouse fibroblasts grown in tissue culture. In this 

 case, the sequential appcai-ance of enzymes was observed upon transfer 

 of cells to fresh medium. Finally, it has been shown that production of 

 these enzymes by normal liver can be induced by injecting animals with 

 thymidine (Hiatt and Bojarski, 19()0). Tims, it would appear that a 

 sequence of enzymatic steps, or rather a metabolic pathway, is con- 

 cerned and that this pathway may be normally repressed or inhibited. 

 In this connection, it is interesting to note that nomial liver contains 

 an extractable substance which will specifically inhilnt the tliyniidylic 

 kinase system (Gray et al., 1960). 



h. Is the establishment of this pathway the sole controllinsj; factor 

 in DNA biosynthesis? If so, one would expect that the maximum rate of 

 DNA synthesis would not be achieved until the entire enzymatic 

 mechanism for the synthesis of thymidine triphosphate was optimally 

 established. Instead, evidence exists which indicates that the maxinnmi 

 enzyme activity (both thymidylic kinase and polymerase) is reached 

 after the rate of DNA synthesis has already begun to decline (Bollum 

 and Potter, 1959; Bianchi et nl, 1961). Similarly, when DNA synthesis 

 ceases in regenerating tissue, precursors continue to accumulate (Wil- 

 liamson and Gaschlbauer, 1961). Thus, if synthesis of thymidine tri- 

 phosphate is responsible for the onset of DNA synthesis, the latter does 

 not cease as a result of an absence of the former. 



Canellakis et al. (1959) observed the presence of enzymes in normal 

 liver, which degrade uridine or thymidine compounds. A reduction in 

 their activity in regenerating liver was thought to contribute to the 

 conti'ol of DNA synthesis. In the same system an increase in thymidylate 

 kinase activity was observed. However, the observed changes in activi- 

 ties of all of these enzymes were not cyclic, whereas DNA synthesis 

 took place in three cycles occurring at 24, 48, and 72 hours. Clearly, if 

 only the production of these enzymes controlled DNA biosynthesis tlicir 

 activity should also have varied in a cyclic manner. 



From these considerations it would appear that although the jiroduc- 

 tion of enzymes needed for the synthesis of DNA precursors may be 

 necessary, it is not a sufficient condition to explain the degree of control 

 observed. 



c. Are other control mechanisms also operating? Evidence from 

 X-radiation studies has indicated that it is possible to block thymidylate 

 kinase synthesis by irradiating regenerating liver at 6 hours whereas 

 this is impossible at 16 hours. In the latter case, however, DNA synthesis 



