What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Primase Activity: Initiation of DNA Replication

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Primase Activity

Primase activity refers to the enzymatic synthesis of short RNA oligonucleotides that serve as primers for DNA replication. Every DNA polymerase—whether bacterial, archaeal, or eukaryotic—is incapable of initiating de novo synthesis; each requires a pre-existing free 3'-hydroxyl group onto which it can add deoxyribonucleotide triphosphates (dNTPs). Primase provides this essential 3'-OH by polymerizing ribonucleotide triphosphates (rNTPs) complementary to a DNA template strand, generating a short RNA primer that is subsequently extended by DNA polymerase.

The fundamental importance of primase activity cannot be overstated: without it, DNA replication could not begin at origins of replication, and the discontinuous synthesis of the lagging strand would be impossible. The enzyme acts as a molecular bridge between the unwound, single-stranded DNA template and the DNA polymerases that carry out bulk genome duplication. This article provides a comprehensive examination of primase activity, from its biochemical mechanism to its regulation, structural biology, and clinical relevance.

Role in DNA Replication

DNA replication proceeds bidirectionally from origins of replication, where the double helix is unwound by helicase enzymes to expose single-stranded DNA (ssDNA) templates. Both leading and lagging strand synthesis require primers. The leading strand requires only a single primer at the origin, after which DNA polymerase synthesizes continuously in the 5'→3' direction. The lagging strand, however, is synthesized discontinuously as Okazaki fragments, each of which requires its own primer. In Escherichia coli, Okazaki fragments are approximately 1,000–2,000 nucleotides long, meaning that primase must initiate synthesis frequently—roughly once every 1–2 seconds during rapid growth.

The RNA primers synthesized by primase are typically 8–12 nucleotides in length in bacteria and 8–14 nucleotides in eukaryotes. These primers are later removed by specialized nucleases (RNase H and flap endonuclease 1, FEN1) and replaced with DNA by DNA polymerase, after which DNA ligase seals the remaining nick. The transient nature of RNA primers underscores their role as initiation factors rather than permanent components of the genome.

Historical Discovery

The existence of primase activity was first demonstrated in the late 1960s and early 1970s through studies of bacteriophage systems. In 1968, it was observed that DNA polymerase I from E. coli could not initiate synthesis on single-stranded templates without a primer. The search for the enzyme responsible for primer synthesis culminated in the identification of the dnaG gene product in E. coli as a distinct RNA polymerase activity in 1972. Subsequent work by Alberts and colleagues on the T4 bacteriophage replication system revealed a similar primase activity associated with the gene 61 protein, now known as gp61. These foundational studies established that primase is a specialized RNA polymerase dedicated to primer synthesis, distinct from the RNA polymerases involved in transcription.

The Mechanism of Primase Activity

Primase activity proceeds through a series of ordered biochemical steps that culminate in the production of a short RNA primer. Understanding this mechanism requires examination of substrate recognition, catalysis, and the termination decision.

Substrate Recognition

Primase binds to single-stranded DNA with a preference for specific sequences. In bacteria, DnaG primase recognizes a trinucleotide sequence, typically 5'-CTG-3' or 5'-CAG-3', although the exact consensus varies. The enzyme binds this sequence through its RNA polymerase-like catalytic domain, positioning the template so that the first nucleotide incorporated is complementary to the base immediately downstream of the recognition site.

Eukaryotic primase, the p48/p58 heterodimer within the Pol α/primase complex, shows less stringent sequence specificity but still exhibits a preference for pyrimidine-rich templates. The initial NTP is typically ATP or GTP, and the first two nucleotides form a stable dinucleotide that is resistant to pyrophosphorolysis, committing the enzyme to primer synthesis.

The interaction between primase and its template is stabilized by the zinc-binding domain, which contacts the DNA backbone and positions the template strand in the active site. This domain is essential for sequence-specific recognition; mutations that disrupt zinc coordination abolish primase activity entirely.

Catalytic Mechanism

Primase catalyzes the polymerization of rNTPs in the 5'→3' direction, using the same two-metal-ion mechanism employed by all nucleic acid polymerases. The reaction proceeds as follows:

Template binding: Primase binds ssDNA at the recognition sequence, with the 3' end of the template positioned in the active site.
Initiation: The enzyme selects the first rNTP complementary to the template base and positions it in the catalytic site. A second rNTP is then aligned, and a phosphodiester bond is formed between the 5'-phosphate of the incoming nucleotide and the 3'-OH of the first nucleotide. This step is slow and rate-limiting, which is why primase activity is measured in nucleotides per minute rather than per second.
Processive elongation: Once the dinucleotide is formed, primase undergoes a conformational change that increases its processivity. Subsequent rNTPs are added sequentially, with the enzyme translocating along the template after each incorporation.
Termination: After synthesizing a primer of characteristic length, primase transfers the RNA primer to DNA polymerase (or dissociates, depending on the system). The mechanism of termination is not fully understood but appears to involve product length sensing and structural constraints of the active site.

The catalytic rate of primase is notably slow compared to DNA polymerases. Bacterial DnaG synthesizes primers at approximately 10–50 nucleotides per minute, whereas DNA polymerase III holoenzyme extends DNA at approximately 1,000 nucleotides per second. This disparity reflects the fact that primase must carefully select and position each incoming rNTP without the benefit of proofreading.

Primer Length and Termination

Primer length is tightly controlled and species-specific. Bacterial primers are typically 8–12 nucleotides, eukaryotic primers 8–14 nucleotides, and archaeal primers 7–10 nucleotides. The termination mechanism involves a "counting" function within the primase itself. Structural studies of eukaryotic primase have revealed that the p58 subunit acts as a ruler, physically blocking further elongation once the primer reaches a critical length. In bacterial DnaG, the N-terminal domain is thought to play a similar role.

After primer synthesis, the primer is transferred to DNA polymerase. In bacteria, DnaG interacts directly with the DnaB helicase and the χ subunit of DNA polymerase III holoenzyme, facilitating handoff. In eukaryotes, the p180 catalytic subunit of Pol α binds the RNA primer and extends it with DNA (approximately 20 nucleotides) before the entire RNA-DNA hybrid is transferred to the processive polymerases Pol ε (leading strand) or Pol δ (lagging strand).

Primase Structure and Functional Domains

Primases across all domains of life share a core catalytic architecture that resembles a right hand, with thumb, palm, and fingers domains, despite limited primary sequence conservation. The active site contains three conserved aspartate residues that coordinate two divalent metal ions (typically Mg²⁺ or Mn²⁺) essential for catalysis.

Archaeal and Eukaryotic Primases

Eukaryotic and archaeal primases belong to the same protein family and share a heterodimeric architecture. The eukaryotic primase consists of two subunits: p49 (also called PRIM1 in humans) and p58 (PRIM2). The p49 subunit contains the catalytic active site, while p58 provides structural stability, contributes to template binding, and regulates primer length. The p58 subunit also contains an iron-sulfur [4Fe-4S] cluster that is essential for primase activity, although its precise role remains under investigation.

The catalytic subunit (p49) contains three conserved motifs (I, II, and III) that form the active site. Motif I contains the metal-binding aspartates, motif II contributes to NTP binding, and motif III is involved in template recognition. The zinc-binding domain is located at the N-terminus of p49 and is connected to the catalytic core by a flexible linker.

Archaeal primases are structurally simpler, often existing as a single catalytic subunit with a small accessory subunit. The archaeal enzyme from Pyrococcus furiosus has been extensively characterized and serves as a model for understanding the eukaryotic enzyme.

Bacterial Primase (DnaG)

Bacterial primase DnaG has a modular architecture distinct from eukaryotic primases. The protein consists of three domains:

N-terminal zinc-binding domain (ZBD): Responsible for sequence-specific template recognition. This domain contains a zinc ribbon motif that binds the trinucleotide recognition sequence.
Central RNA polymerase domain (RPD): Contains the catalytic active site with the conserved aspartate residues. This domain adopts a fold similar to the catalytic domain of eukaryotic primases, despite minimal sequence homology.
C-terminal helicase-binding domain (HBD): Mediates interaction with the DnaB replicative helicase. This domain is essential for recruiting DnaG to the replication fork and for stimulating primase activity.

The three domains are connected by flexible linkers that allow the protein to adopt different conformations during the catalytic cycle. The HBD is also involved in the interaction with DNA polymerase III, facilitating primer handoff.

Regulation of Primase Activity

Primase activity must be tightly regulated to ensure that primers are synthesized at the correct time, in the correct location, and in appropriate numbers. Dysregulation of primase leads to aberrant replication, genomic instability, and cell death.

Interaction with Helicase

The most critical regulatory interaction is between primase and the replicative helicase. In bacteria, DnaG binds to DnaB through its C-terminal domain. This interaction serves multiple functions:

Recruitment: DnaB helicase unwinds DNA at the replication fork and recruits DnaG to the exposed ssDNA. Without this interaction, DnaG cannot access the template efficiently.
Stimulation: DnaB binding stimulates DnaG activity by increasing its affinity for ssDNA and rNTPs. In vitro, DnaG alone has low basal activity; addition of DnaB stimulates primer synthesis 10- to 20-fold.
Regulation of primer frequency: The DnaB-DnaG interaction is dynamic, with DnaG binding transiently to DnaB to synthesize a primer and then dissociating. The frequency of primer synthesis on the lagging strand is determined by the rate of DnaG-DnaB association and dissociation, which is in turn influenced by the concentration of DnaG and the processivity of the helicase.

In eukaryotes, the analogous interaction occurs between the Pol α/primase complex and the CMG helicase (Cdc45-MCM-GINS). The primase subunit p58 interacts with the MCM10 protein, which recruits Pol α/primase to the replication fork and stabilizes its association with chromatin. MCM10 also stimulates primase activity and promotes primer handoff to Pol ε and Pol δ.

Cell Cycle Control

Primase activity is regulated during the cell cycle to ensure that replication occurs only during S phase. In eukaryotes, the Pol α/primase complex is phosphorylated by cyclin-dependent kinases (CDKs) and other kinases, which modulates its activity and interactions.

CDK phosphorylation: CDK2-cyclin E and CDK2-cyclin A phosphorylate the p180 subunit of Pol α during S phase. This phosphorylation is required for efficient primer synthesis and for the interaction of Pol α/primase with other replication factors.
Cdc7-Dbf4 kinase (DDK): DDK phosphorylates MCM proteins and is required for origin firing. Although DDK does not directly phosphorylate primase, its activity is required for the recruitment of Pol α/primase to origins.
Checkpoint regulation: In response to DNA damage, the ATR/Chk1 checkpoint pathway inhibits origin firing and reduces primase activity. This is achieved in part through phosphorylation of MCM10 and other replication factors, which impairs their interaction with primase.

In bacteria, primase activity is not cell-cycle regulated in the same way, but its expression is coordinated with the replication cycle. The dnaG gene is part of the macromolecular synthesis operon and is transcribed at higher levels during rapid growth when more replication origins are active.

Primase in Different Domains of Life

While the fundamental role of primase is conserved, the enzyme has evolved distinct features in bacteria, archaea, and eukaryotes. These differences reflect the different replication strategies and regulatory requirements of each domain.

Bacterial DnaG

Bacterial primase is a single polypeptide (DnaG) that functions as a monomer. It is recruited to the replication fork through its interaction with DnaB helicase. The enzyme synthesizes primers of 8–12 nucleotides with a preference for the sequence 5'-CTG-3' on the template strand. Bacterial primase is relatively non-processive, synthesizing a single primer per binding event before dissociating.

The dnaG gene is essential in all bacteria studied to date. Conditional lethal mutants have been isolated in E. coli, and these mutants arrest DNA replication at the non-permissive temperature, demonstrating the essential nature of primase activity.

Eukaryotic Pol α/primase

Eukaryotic primase is part of a four-subunit complex: Pol α/primase, consisting of p180 (catalytic DNA polymerase), p68 (regulatory subunit), p58 (primase accessory), and p49 (primase catalytic). The primase activity resides in the p49/p58 heterodimer, while the p180/p68 heterodimer provides DNA polymerase activity that extends the RNA primer with DNA.

The eukaryotic enzyme is more complex than its bacterial counterpart, reflecting the greater regulatory demands of eukaryotic replication. The primase activity is stimulated by the p180 subunit, which also protects the RNA primer from degradation. The p68 subunit is required for nuclear import and for the interaction with the replication protein A (RPA), which coats ssDNA at the replication fork.

Archaeal Primase

Archaeal primases are structurally similar to eukaryotic primases but are often simpler in composition. Most archaea encode a single catalytic primase subunit (PriS) and a single accessory subunit (PriL), which are homologous to p49 and p58, respectively. Some archaea also encode a second primase, PriX, which is involved in specific replication contexts.

Archaeal primases are of particular interest because they are more closely related to eukaryotic primases than to bacterial DnaG, making them useful models for studying the eukaryotic enzyme. Additionally, some archaeal primases exhibit DNA polymerase activity in addition to primase activity, a feature that has been exploited for biotechnological applications.

The following table summarizes the key features of primases across the three domains of life:

Feature	Bacterial (DnaG)	Eukaryotic (Pol α/primase)	Archaeal (PriS/PriL)
Subunit composition	Single subunit	Four subunits (p180, p68, p58, p49)	Two subunits (PriS, PriL)
Primer length	8–12 nt	8–14 nt	7–10 nt
Template specificity	5'-CTG-3'	Low sequence specificity	Low sequence specificity
Helicase partner	DnaB	CMG complex	MCM (likely)
Associated DNA polymerase	Pol III (via χ subunit)	Pol α (p180)	PriS itself (in some species)
Proofreading	None	None	None
Metal cofactors	Mg²⁺	Mg²⁺, [4Fe-4S] cluster	Mg²⁺, [4Fe-4S] cluster

Experimental Methods to Study Primase Activity

Studying primase activity requires specialized assays that can detect the synthesis of short RNA products. These methods range from classical radioactive assays to modern fluorescence-based approaches.

In Vitro Primase Assays

The standard primase assay measures the incorporation of radiolabeled rNTPs into acid-precipitable RNA products. A typical reaction contains:

50 mM Tris-HCl (pH 7.5)
10 mM MgCl₂
1 mM DTT
100 µM each of ATP, GTP, CTP, and UTP (with one radiolabeled, typically [α-³²P]ATP or [α-³²P]GTP)
100 ng of single-stranded DNA template (e.g., M13 ssDNA)
10–100 nM primase

The reaction is incubated at 37°C (or the optimal temperature for the enzyme being studied) for 10–30 minutes, then stopped by the addition of EDTA. Products are precipitated with trichloroacetic acid (TCA), collected on glass fiber filters, and quantified by scintillation counting.

For more detailed analysis, primers are resolved by denaturing polyacrylamide gel electrophoresis (PAGE). Products are visualized by autoradiography or phosphorimaging, and the length distribution of primers can be determined. This approach reveals the characteristic primer length and any processivity defects in mutant enzymes.

Mutational Analysis

Site-directed mutagenesis is a powerful tool for dissecting primase mechanism. Conserved residues in the active site, zinc-binding domain, and accessory subunits can be mutated to assess their contributions to catalysis, template recognition, and primer length control.

For example, mutation of any of the three conserved aspartate residues in the catalytic site abolishes primase activity, confirming their essential role in metal coordination. Mutations in the zinc-binding domain of DnaG eliminate sequence-specific recognition but do not affect the ability to synthesize primers on non-specific templates, demonstrating the separation of template recognition from catalysis.

Structural Studies

X-ray crystallography and cryo-electron microscopy (cryo-EM) have provided high-resolution structures of primases from all domains of life. These structures have revealed the architecture of the active site, the conformational changes associated with primer synthesis, and the molecular basis of primer length control.

Key structures include:

The crystal structure of E. coli DnaG catalytic domain (PDB 1DDE), which revealed the RNA polymerase-like fold.
The structure of the eukaryotic Pol α/primase complex (PDB 4RR3), which showed how the p58 subunit regulates primer length.
Cryo-EM structures of the CMG-Pol α/primase complex, which have illuminated the architecture of the eukaryotic replication fork.

Structural studies have also been instrumental in understanding how primase inhibitors bind and block enzyme activity, informing drug development efforts.

Primase Activity and Disease

Given the essential role of primase in DNA replication, it is not surprising that defects in primase activity are associated with human disease. Additionally, primase has emerged as a promising target for anticancer therapy.

Primase Mutations in Disease

Mutations in the genes encoding primase subunits have been linked to several human disorders:

PRIM1 mutations: Biallelic mutations in PRIM1 (encoding p49) cause a rare syndrome characterized by microcephaly, growth retardation, and bone marrow failure. These mutations typically reduce but do not eliminate primase activity, leading to impaired replication in rapidly dividing tissues.
PRIM2 mutations: Mutations in PRIM2 (encoding p58) have been associated with similar phenotypes, including microcephaly and developmental delay. Some PRIM2 mutations affect the iron-sulfur cluster, disrupting the stability of the p49/p58 heterodimer.
Somatic mutations in cancer: Somatic mutations in PRIM1 and PRIM2 have been identified in various cancers, although their functional significance is not always clear. Some mutations may confer a growth advantage by altering the fidelity or processivity of replication.

In addition to germline mutations, altered expression of primase subunits has been observed in cancer. Overexpression of PRIM1 has been reported in several tumor types and is associated with poor prognosis, possibly reflecting increased replicative demand in rapidly dividing cells.

Inhibitors as Anticancer Agents

The essential role of primase in DNA replication makes it an attractive target for anticancer therapy. Inhibiting primase would block both leading and lagging strand synthesis, leading to replication fork collapse and cell death.

Several natural products and synthetic compounds have been identified as primase inhibitors:

Aphidicolin: Although primarily known as an inhibitor of DNA polymerase α, aphidicolin also inhibits primase activity at higher concentrations. Its clinical use is limited by toxicity.
6-anilinouracil derivatives: These compounds inhibit bacterial primase and are being developed as antibacterial agents. They show selectivity for bacterial DnaG over eukaryotic primase.
Tryptamine derivatives: Certain tryptamine-based compounds have been shown to inhibit eukaryotic primase activity in vitro, although their in vivo efficacy remains to be established.

The development of selective primase inhibitors for cancer therapy faces the challenge of achieving selectivity for the tumor cell enzyme while sparing normal tissues. Because primase is essential in all dividing cells, systemic inhibition would be expected to cause significant toxicity. However, the differential expression of primase in cancer cells versus normal cells may provide a therapeutic window.

Common Pitfalls and Misconceptions

Students frequently encounter difficulties when learning about primase activity. The following sections address the most common misconceptions and errors.

Primase vs. DNA Polymerase

A frequent error is confusing primase with DNA polymerase. While both enzymes synthesize nucleic acid polymers in the 5'→3' direction, they differ in several fundamental ways:

Substrate: Primase uses ribonucleotide triphosphates (rNTPs); DNA polymerase uses deoxyribonucleotide triphosphates (dNTPs).
Initiation: Primase can initiate synthesis de novo on a template; DNA polymerase requires a pre-existing primer.
Processivity: Primase synthesizes short products (8–14 nucleotides) and is poorly processive; DNA polymerase synthesizes long products (thousands of nucleotides) and is highly processive.
Proofreading: Primase lacks proofreading activity; many DNA polymerases possess 3'→5' exonuclease activity.

The distinction is summarized in the Primase vs Polymerase comparison.

Why RNA Primers?

Students often ask why RNA primers are used instead of DNA primers. The answer lies in the evolutionary history and biochemical constraints of DNA replication:

De novo initiation: DNA polymerases cannot initiate synthesis without a primer. Using RNA as the primer allows the cell to use a dedicated enzyme (primase) that is specialized for de novo initiation.
Fidelity: RNA primers are transient and are removed and replaced with DNA. This provides an additional opportunity to correct errors that might have been introduced during primer synthesis.
Discrimination: The presence of RNA in the nascent strand may help distinguish the newly synthesized strand from the template during mismatch repair and other processes.

The use of RNA primers is a universal feature of DNA replication, from bacteriophages to humans, underscoring its fundamental importance.

Primase Fidelity

Another common misconception is that primase must be highly accurate because errors in primers would be propagated into the genome. In reality, primase is a relatively error-prone enzyme, with error rates estimated at 10⁻³ to 10⁻⁴ per nucleotide incorporated—several orders of magnitude higher than the error rate of replicative DNA polymerases (10⁻⁹ to 10⁻¹⁰).

This low fidelity is tolerated because:

RNA primers are removed and replaced with DNA, providing an opportunity to correct errors.
The primers are short, limiting the number of errors per replication event.
The errors that do occur are typically in non-coding regions or are corrected by downstream repair pathways.

Primase also lacks the ability to discriminate between rNTPs and dNTPs with high precision, although it strongly prefers rNTPs. This is in contrast to DNA polymerases, which are highly selective for dNTPs.

Overlooking the Lack of Proofreading

Students sometimes assume that all polymerases proofread their products. This is incorrect. Primase has no 3'→5' exonuclease activity and cannot remove misincorporated nucleotides. Once a nucleotide is added to the growing primer, it remains in place. This lack of proofreading is a defining feature of primase and contributes to its relatively low fidelity.

Summary and Key Takeaways

Primase activity is the enzymatic synthesis of short RNA primers that provide the free 3'-OH required for DNA polymerase to initiate DNA synthesis. This activity is essential for both leading and lagging strand replication and is conserved across all domains of life.

The enzyme operates through a mechanism involving template binding, NTP selection, dinucleotide formation, and processive elongation, terminating after a characteristic primer length. Primase lacks proofreading activity and is relatively error-prone, but this is tolerated because primers are transient and removed during maturation.

Primase structure varies across domains: bacterial DnaG is a single subunit, eukaryotic primase is a heterodimer within the Pol α/primase complex, and archaeal primase is a simpler heterodimer. Regulation occurs through interactions with helicases, accessory proteins, and post-translational modifications.

Defects in primase activity cause human disease, and primase inhibitors are being explored as therapeutic agents. Understanding primase activity is fundamental to understanding DNA replication and its regulation.

Frequently Asked Questions

What is primase activity?

Primase activity is the enzymatic synthesis of short RNA oligonucleotides (primers) complementary to a DNA template. These primers provide the free 3'-hydroxyl group required by DNA polymerases to initiate DNA synthesis. Primase is a specialized RNA polymerase that can initiate synthesis de novo, unlike DNA polymerases which require a pre-existing primer.

Why does primase synthesize RNA primers instead of DNA?

Primase synthesizes RNA primers because DNA polymerases cannot initiate de novo synthesis; they require a free 3'-OH to extend. RNA primers are transient—they are removed and replaced with DNA during replication maturation—which provides an opportunity to correct errors introduced during primer synthesis. The use of RNA also helps distinguish newly synthesized strands during repair processes.

Does primase have proofreading activity?

No. Primase lacks 3'→5' exonuclease activity and cannot remove misincorporated nucleotides. This makes primase relatively error-prone, with error rates of approximately 10⁻³ to 10⁻⁴ per nucleotide. The lack of proofreading is tolerated because primers are short and are removed and replaced with DNA during replication.

How is primase activity regulated?

Primase activity is regulated through multiple mechanisms: interaction with the replicative helicase (DnaB in bacteria, CMG in eukaryotes) which recruits and stimulates primase; accessory proteins such as MCM10 that stabilize primase at the fork; and post-translational modifications including phosphorylation by CDKs and other kinases that modulate activity during the cell cycle.

What is the difference between primase and DNA polymerase?

Primase synthesizes RNA primers using rNTPs and can initiate synthesis de novo, while DNA polymerase synthesizes DNA using dNTPs and requires a pre-existing primer. Primase is poorly processive and lacks proofreading, whereas replicative DNA polymerases are highly processive and possess proofreading activity. See Primase vs Polymerase for a detailed comparison.

What happens if primase is inhibited?

Inhibition of primase blocks both leading and lagging strand DNA synthesis, leading to replication fork collapse and cell death. In bacteria, primase inhibitors are bactericidal. In eukaryotes, primase inhibition is toxic to dividing cells, which is the basis for exploring primase inhibitors as anticancer agents.

Is primase activity the same in all organisms?

The fundamental mechanism is conserved, but the enzyme structure and regulation differ across domains. Bacterial primase (DnaG) is a single subunit with sequence-specific template recognition. Eukaryotic primase is a heterodimer (p49/p58) within the four-subunit Pol α/primase complex. Archaeal primase is a simpler heterodimer (PriS/PriL). Primer lengths also vary: 8–12 nucleotides in bacteria, 8–14 in eukaryotes, and 7–10 in archaea.

Key Takeaways

Primase activity is the synthesis of short RNA primers that provide the essential 3'-OH for DNA polymerase to initiate DNA replication.
Primase is a specialized RNA polymerase that can initiate synthesis de novo, unlike DNA polymerases which require primers.
The mechanism involves template binding, NTP selection, slow dinucleotide formation, and processive elongation to a characteristic primer length.
Primase lacks proofreading activity and is error-prone, but this is tolerated because primers are transient and removed during replication.
Primase structure and regulation differ across bacteria (DnaG), eukaryotes (Pol α/primase), and archaea (PriS/PriL), reflecting different replication strategies.
Primase activity is regulated by helicase interactions, accessory proteins, and cell-cycle-dependent phosphorylation.
Defects in primase cause human disease, and primase inhibitors are being developed as antibacterial and anticancer agents.