A complex containing the 80,000-dalton precursor to the adenovirus (Ad)-encoded terminal protein (pTP) and a 140,000-dalton protein is required for Ad DNA replication in vitro. This complex has been separated into subunits by glycerol gradient centrifugation in the presence ofurea. The isolated 140,000-dalton subunit contains a DNA polymerase activity which can be differentiated from all host DNA polymerases. No enzyme activity was detected with the isolated pTP. The requirements for reactions involved in the initiation of Ad DNA replication were determined by using the isolated subunits. The covalent addition ofdCMP, the first nucleotide in the DNA chain, to the pTP, which serves as the primer for replication, required the DNA polymerase subunit as well as the pTP. Synthesis of viral DNA in vitro also required both subunits. The properties of the DNA polymerase suggest that it may be a viral gene product.Studies with in vitro adenovirus (Ad) DNA replication systems have provided evidence in support of a model in which the virus-encoded 80,000-dalton pre-terminal protein (pTP) serves as the primer for the initiation of Ad DNA replication (1). According to the model, replication begins with the covalent addition of dCMP, the 5'-terminal nucleotide of each Ad DNA strand, to the pTP. The product of this reaction, pTP-dCMP, has a 3'-OH which can be used by a DNA polymerase as a primer terminus, allowing elongation ofthe nascent strand. The evidence favoring this model includes the observation that all nascent DNA chains are covalently linked to protein (2). In addition, the predicted intermediate, pTP-dCMP, is synthesized in vitro (3).Synthesis of pTP-dCMP provided an assay which permitted the purification ofpTP from cytoplasmic extracts ofAd-infected HeLa cells (4, 5). With the purified protein, the covalent addition ofdCMP to the pTP required MgCl2 and a DNA cofactor which could be either Ad DNA with terminal protein covalently linked to the 5' terminus ofeach strand (Ad DNA-pro) or a number of single-stranded DNAs, such as OX174 DNA (6). With Ad DNA-pro as a cofactor, the reaction was stimulated as much as 10-fold by ATP and a nuclear extract of uninfected cells; the virus-encoded DNA binding protein (Ad DBP) had no effect (4). In contrast, when single-stranded 4X174 DNA was used, the reaction was inhibited by ATP, nuclear extract, or Ad DBP. The pTP copurified with a DNA polymerase activity (Ad DNA polymerase) which resembled DNA polymerase a in its template preference and sensitivity to certain inhibitors. However, the Ad DNA polymerase was insensitive to aphidicolin, a potent inhibitor of DNA polymerase a. The most purified fraction of pTP contained approximately equimolar quantities of pTP and a 140,000-dalton protein. Because DNA polymerase a obtained from several sources has been reported to have a catalytic subunit of 140,000-160,000 daltons (7,8), it seemed likely that the 140,000-dalton protein would be the catalytic subunit of the Ad DNA polymerase. The sedimentation behavior of the purified pT...