Nuclear extracts from adenovirus type 5 (Ad5) infected HeLa cells were used to study the template requirements for adenovirus DNA replication in vitro. When XbaI digested Ad5 DNA, containing the parental terminal protein (TP), was used as a template preferential synthesis of the terminal fragments was observed. The newly synthesized DNA was covalently bound to the 82 kD preterminal protein (pTP). Plasmid DNAs containing the Ad2 origin sequence or the Ad12 origin sequence with small deletions were analyzed for their capacity to support pTP-primed DNA replication. Circular plasmid DNAs were inactive. When plasmids were linearized to expose the adenovirus origin, both Ad2 and Ad12 TP-free fragments could support initiation and elongation similarly as Ad5 DNA-TP, although with lower efficiency. These observations indicate that the parental terminal protein is dispensable for initiation in vitro. The presence of 29 nucleotides ahead of the molecular end or a deletion of 14 base pairs extending into the conserved sequence (9-22) destroyed the template activity. DNA with a large deletion within the first 8 base pairs could still support replication while a small deletion could not. The results suggest that only G residues at a distance of 4-8 nucleotides from the start of the conserved sequence can be used as template during initiation of DNA replication.

Initiation of adenovirus DNA replication is dependent on a complex of the precursor of the terminal protein and the adenovirus-coded DNA polymerase (pTP-pol complex). This complex catalyzes the formation of a covalent linkage between dCMP and pTP in the presence of a functional origin of DNA replication residing in the terminal nucleotide sequence of adenovirus DNA. We have purified the pTP-pol complex of adenovirus type 5 and studied its binding to double-stranded DNA. Using DNA-cellulose chromatography it could be shown that the pTP-pol complex has a higher affinity for adenovirus DNA than for calf thymus or pBR322 DNA. From the differential binding of the pTP-pol complex to plasmids containing adenovirus terminal sequences with different deletions, it has been concluded that a sequence of 14 nucleotide pairs at positions 9-22 plays a crucial role in the binding of pTP-pol to adenovirus DNA. This region is conserved in the DNA's of all human adenovirus serotypes and is obviously an important structural element of the adenovirus origin of DNA replication. Comparative binding studies with adenovirus DNA polymerase and pTP-pol indicated that pTP is responsible for the binding. The nature of the binding of pTP-pol to the conserved sequence will be discussed.

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