DnaG

DnaG is the bacterial DNA primase, an RNA polymerase that synthesizes short RNA primers required to initiate DNA synthesis by DNA polymerase III during chromosome replication. In many bacteria, including Escherichia coli, the primase is encoded by the dnaG gene and provides primers for both lagging- and, to a lesser extent, leading-strand synthesis. Primer synthesis is essential for transitioning from DNA unwinding to DNA synthesis on the replication fork.

Structure and interactions: DnaG is a roughly 60-kDa protein composed of three regions. An N-terminal zinc-binding domain mediates interactions with the DnaB helicase. A central RNA polymerase–like catalytic domain carries out primer synthesis. A C-terminal domain participates in stable interactions within the primosome and with the DNA polymerase III holoenzyme, facilitating primer handoff after synthesis. DnaG forms the primosome together with DnaB helicase; the helicase stimulates primase activity and provides the single-stranded template.

Mechanism and specificity: DnaG binds exposed single-stranded DNA at the replication fork and catalyzes RNA primer synthesis in the 5′ to 3′ direction, producing primers typically about 8–12 nucleotides long with a 5′ triphosphate. These primers are extended by DNA polymerase III to begin Okazaki fragment synthesis on the lagging strand, while leading-strand initiation also relies on primer formation.

Genetics and evolution: The dnaG gene is essential for bacterial growth in most species. DnaG-type primases are distinct from archaeal/eukaryotic PriS/PriL primases and are characteristic of bacteria. Variations exist among species in the architecture and regulatory interactions of the primosome.

Clinical relevance: DnaG is a potential target for antibacterial strategies. Structural and biochemical studies of DnaG and its interactions with DnaB have informed understanding of primer synthesis and replication coordination.