Enzymatic Synthesis of Deoxyribonucleic Acid

Abstract

Abstract DNA polymerase induced in Escherichia coli by phage T4 am N82 has been prepared in a highly purified state. The molecular weight of about 112,000 for this enzyme compares with a value of 109,000 for the host (E.coli) DNA polymerase. The amino acid compositions of the two enzymes, however, are distinctive, especially the half-cystine value of 15 residues for the phage enzyme and 3 for the bacterial enzyme. The purified phage enzyme contains an exonuclease activity which is physically inseparable from the polymerase activity but is suppressed or obscured by conditions favorable for replication. The polymerase requires a single stranded template, with a free 3'-hydroxyl terminus, which is replicated to an extent that approaches but never exceeds the DNA input. The product is a helical structure in which the newly synthesized strand is covalently linked from its 5'-terminus to the 3'-terminus of the template. Various features of the replication process and the product suggest a mechanism in which the 3'-terminus of the template first loops back upon itself and then serves as the priming end for replication of the remainder of the template. The evidence indicates that the phage polymerase, unlike the E. coli enzyme, is unable to initiate new strands or utilize a fully helical DNA as template.