A multienzyme system for priming the replication of phiX174 viral DNA

Synthesis of the oligonucleotides that prime replication of phiX174 single-stranded DNA employs complex protein machinery of the host cell which is probably used by the cell to replicate its own chromosome. Primer synthesis depends on at least five proteins (DNA binding protein, dnaB and dnaC proteins, protein i, and protein n) and ATP to form a replication intermediate and another protein, primase (dnaG protein), to assemble the oligonucleotide by template transcription. The data in this paper show that ribo- and deoxyribonucleoside triphosphates can serve as substrates and form hybrid primers when present together. Both RNA and DNA primers were initiated with ATP. At least three of the four base-pairing nucleoside triphosphates were required for the transcription that generates effective primers. Over 90% of the RNA and DNA transcripts were extended into complementary strands by DNA polymerase III holoenzyme. At optimal triphosphate concentrations, the rate and extent of primer formation were greater from ribonucleoside triphosphates than from deoxyribonucleoside triphosphates. Uncoupled from DNA replication, the length of RNA primers was 14 to 50 residues, the DNA primers 4 to 20 residues. The fingerprint pattern of an RNase digest of RNA primers has a complexity suggestive of transcription from many sites on the phiX174 template. The multienzyme priming system is highly specific for phiX174 DNA as template.