dnaG gene product, a rifampicin-resistant RNA polymerase, initiates the conversion of a single-stranded coliphage DNA to its duplex replicative form.

The protein responsible for the initiation of conversion of single-stranded phage G4 DNA to the duplex replicative form has been purified approximately 3000-fold and identified with Escherichia coli dnaG gene product. The protein is a rifampicin-resistant RNA polymerase of approximately 64,000 daltons. It catalyzes the incorporation of the four ribonucleoside triphosphates into an oligoribonucleotide, using as template the single-stranded DNA coated with the DNA unwinding protein of E. coli. An RNA transcript of a unique region of the chromosome can serve as a primer by covalent extension by DNA polymerase III holoenzyme to form a nearly full-length linear complementary strand. A similar role for the dnaG protein in the initiation of nascent (Okazaki) fragments in replication of the host chromosome is discussed.