VITAMINS, ANTIBIOTICS, AND GROWTH 351 



purified medium was produced by thymidine or by tetrahydrofolic acid 

 and its formylated derivatives, but folic acid would not produce normal 

 growth except at very high levels. This can perhaps be explained on the 

 basis of the organism being deficient in dihydrofolic reductase, so that 

 the organism's small supply of tetrahydrofoHc acid disappears during 

 the synthesis of thymidine, but if thymidine is furnished, then the 

 supply of tetrahydrofolic acid is conserved and is used for the catalytic 

 production of purines and other "single-carbon" metabolites. The same 

 reasoning serves to explain the effect of thymidine in reversing the in- 

 hibiting effect of aminopterin for E. coli. 



Studies of isolated enzyme systems are essential to further under- 

 standing of the processes of growth. 



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