Primases

Primases are enzymes that synthesize short RNA primers required to start DNA synthesis during replication. They are integral components of the replication machinery and function alongside helicases to form the replication fork. In bacteria, the primase is a single catalytic subunit called DnaG, which operates as part of the primosome complex that includes the DnaB helicase. The RNA primer produced by bacterial primase is short, typically around ten nucleotides, and provides the starting point for DNA polymerase III to extend. In archaea and eukaryotes, primase is a two-subunit enzyme (often referred to as PriS and PriL) that forms a functional complex with DNA polymerase alpha (the Pol α-primase complex). This arrangement synthesizes a primer that is then handed off to the main replicative polymerases (Pol III in bacteria; Pol δ/Pol ε with Pol α in eukaryotes) to continue DNA synthesis. Primases can initiate primers on both leading- and lagging-strand templates, with a prominent role in initiating Okazaki fragments on the lagging strand.

After primer synthesis, RNA segments are removed and replaced with DNA. In bacteria, DNA polymerase I removes the RNA primer via its 5' to 3' exonuclease activity and fills the gap with DNA. In eukaryotes, RNase H1 and flap endonuclease 1 (FEN1) excise the RNA primer, and the resulting DNA gaps are filled by DNA polymerases delta or epsilon. Primases are essential for genome replication across domains of life, enabling the proper initiation of DNA synthesis and coordination with the activity of larger replisome complexes.