Abstract
THE work of Kornberg on DNA repair and synthesis1,2 implicates deoxyribonucleoside 5′-triphosphate as a direct precursor of DNA synthesis. This relationship was questioned by the possibility of alternative replication schemes3,4. Werner5 studied the flux of thymine and thymidine into Escherichia coli DNA to determine the in vivo precursors of replicating DNA. Werner studied the incorporation of 3H labelled thymine into DNA and intracellular nucleotide pools under steady state conditions, in which thymine is converted into thymidine, thymidine monophosphate (TMP), thymidine diphosphate (TDP) and thymidine triphosphate (TTP). Werner measured separately the activities of labelled TMP, TDP, TTP and DNA at various times after E. coli cells had been exposed to a 3H-thymine synthetic medium. From a qualitative consideration of his results, Werner concluded that both TDP and TTP—but not TMP—were possible direct precursors of DNA replication.
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