Protein-primed initiation of phage phi 29 DNA replication.

We recently reported the development of an in vitro replication system for bacteriophage phi 29 DNA. We have used this system for the isolation of replication activity associated with gene 3 protein (terminal protein) from phi 29-infected Bacillus subtilis cells. We utilized two assay systems: (i) DNA replication dependent on phi 29 DNA with the 5' end covalently linked to terminal protein (DNA-protein) and (ii) the formation of complex between the terminal protein and dAMP. The DNA-replication and the complex-forming activities were purified together through all steps. The complex of terminal protein and dAMP formed in the purified fraction was shown to serve as an effective primer for successive chain elongation in the presence of dNTPs by a pulse-chase experiment. The protein fraction purified from cells infected with a temperature-sensitive phi 29 mutant in gene 3 was thermolabile compared to the wild-type activity in the assay system for complex formation. This shows that the purified fraction having replication activity includes the gene 3 product of phi 29. Both the DNA replication and the complex formation activities are highly specific for phi 29 DNA-protein as template. The product analysis of elongated DNA revealed that the replication starts at both termini of the phi 29 genome. These results are consistent with the basic elements of the protein-priming model for the initiation of linear DNA synthesis.