Comparative Studies on the Template‐Initiator Requirements of the Chick‐Embryo DNA Polymerase I and the Avian‐Myeloblastosis‐Virus DNA Polymerase

The template requirements of chick embryo DNA polymerase I devoid of nuclease activity, have been investigated. This enzyme catalyzes a repair reaction in vitro. Active templates were obtained by treatment of native DNA by DNAase I or exonuclease III. The DNA polymerase I activity requires divalent cations and is inhibited by high salt concentrations and sulfhydryl group blocking reagents. Homopolydeoxynucleotides dA n , dT n , dC n , are efficient templates in the presence of hydrogen‐bound initiator. The high efficiency of hybrid structures rA n · dT n , dA n · rU n and rI n · dC n in which RNA acts as‐initiator and DNA as template suggests an essential role of this enzyme in DNA replication. The enzyme does not catalyze RNA‐directed DNA synthesis. DNA polymerase I of chick embryo and the avian myeloblastosis virus DNA polymerase were compared on the basis of their activities in the presence of various synthetic primers. Using synthetic DNA · RNA hybrid such as rA n · dT n , the cellular enzyme elongates the RNA strand by copying the DNA template whereas the viral enzyme mainly copies the RNA strand and less efficiently the DNA strand. With polymers of the (I + C) or (G + C) series, both enzymes are equally efficient which dC n as template; the viral enzyme activity can be distinguished from cellular enzymatic activity principally by its capacity to transcribe rC n and less efficiently rI n . The relationship between the responses of these enzymes in vitro to synthetic primers and the assumed physiological function of the cellular and viral DNA polymerases are discussed.