ENZYMATIC SYNTHESIS OF DEOXYRIBONUCLEIC ACID. III. THE INCORPORATION OF PYRIMIDINE AND PURINE ANALOGUES INTO DEOXYRIBONUCLEIC ACID

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Abstract
The deoxyribonucleoside triphosphates of analogs of the pyrimidine and purine bases were prepared chemically and tested as substrates for the enzyme from Escherichia coli which polymerizes deoxyribonucleotides to deoxyribonucleic acid (DNA). Uracil and 5-bromouracil were incorporated into DNA specifically in place of thymine; 5-methyl-and 5-bromocytosine in place of cytosine; and hypoxanthine in place of guanine. Xanthine was not incorporated into DNA. Analysis of the uracil-containing product demonstrated that the uracil was bound in 3[image]-5[image] phosphodiester linkage with each of the deoxyribotides of adenine, guanine, cytosine, and uracil. The specific replacement of the natural bases by these analogs offers additional support for the base-pairing relationships in the double helix proposed by Watson and Crick for the structure of DNA. The existence of kinases in E. coli which phos-phorylate 5-bromodeoxyuridylate to the triphosphate and the apparent absence of such kinases for the phosphorylation of deoxyuridylate are in keeping with the fact that cells incorporate 5-bromouracil, but not uracil, into DNA.