Primase Work on the Lagging Strand: Initiation of Okazaki Fragments
By Dr. Zubair Khalid, DVM, MS, PhD ·
Introduction to DNA Primase and Its Role in Replication
What is Primase?
DNA primase is a specialized RNA polymerase that synthesizes short RNA oligonucleotides complementary to a single-stranded DNA (ssDNA) template. These RNA oligonucleotides, called primers, provide the free 3′-hydroxyl (3′-OH) group that DNA polymerases require to begin DNA synthesis. In all organisms, primase is an essential component of the replisome—the multiprotein machine that carries out DNA replication. In bacteria, primase is the product of the dnaG gene (e.g., DnaG in Escherichia coli), while in eukaryotes, primase exists as a heterodimer of two subunits, PRIM1 (p49) and PRIM2 (p58), which associate with DNA polymerase α (Pol α) to form the Pol α-primase complex. The catalytic activity resides in the small subunit (PRIM1), while the large subunit (PRIM2) contributes to template binding, stability, and regulation.
Primase is unique among nucleic acid polymerases because it can initiate synthesis de novo—that is, it does not require a pre-existing primer or a free 3′-OH to begin polymerization. This property is fundamental to its role in DNA replication, as no DNA polymerase can start a new chain from scratch. The enzyme selects ribonucleoside triphosphates (NTPs) as substrates and incorporates them into a short RNA chain, typically 8–12 nucleotides in length, before dissociating and handing off the primer to a DNA polymerase.
Why Primase is Needed for Lagging Strand Synthesis
The antiparallel structure of double-stranded DNA (dsDNA) creates an inherent asymmetry in replication. DNA polymerases synthesize DNA exclusively in the 5′-to-3′ direction, meaning they add nucleotides to the 3′-OH of a growing chain. At a replication fork, the two template strands are oriented in opposite directions. The leading strand template runs 3′-to-5′ relative to the direction of fork movement, allowing the DNA polymerase to synthesize continuously in the same direction as fork unwinding. The lagging strand template, however, runs 5′-to-3′ relative to fork movement, forcing the polymerase to synthesize DNA in the direction opposite to fork progression. This necessitates a discontinuous mode of synthesis, in which short segments of DNA—Okazaki fragments—are made in a series of steps, each requiring a new RNA primer. Primase therefore acts repeatedly on the lagging strand, generating the primers that initiate each Okazaki fragment. Without primase, the lagging strand could not be replicated, and the replication fork would stall.
The Asymmetry of DNA Replication: Leading vs. Lagging Strand
Directionality of DNA Polymerases
All DNA polymerases share a fundamental mechanistic constraint: they can only add nucleotides to the 3′-OH terminus of a pre-existing nucleic acid chain. This means synthesis proceeds in the 5′-to-3′ direction, reading the template strand in the 3′-to-5′ direction. The chemical basis for this directionality lies in the nucleotidyl transfer reaction, in which the 3′-OH of the growing chain attacks the α-phosphate of an incoming deoxyribonucleoside triphosphate (dNTP), releasing pyrophosphate. The 3′-OH must be present; polymerases cannot create it de novo.
At a replication fork, the helicase unwinds dsDNA into two ssDNA templates. The fork moves in a single direction, but the two templates are antiparallel. For the leading strand, the template is oriented such that the polymerase can move continuously in the same direction as the fork, synthesizing one long, uninterrupted DNA molecule. For the lagging strand, the template is oriented in the opposite direction, so the polymerase must work in a "backstitching" manner, synthesizing short segments away from the fork and then reinitiating closer to the fork as more template is exposed. This asymmetry is a direct consequence of the antiparallel geometry of DNA and the strict 5′-to-3′ directionality of DNA polymerases.
Okazaki Fragments and the Need for Multiple Primers
Okazaki fragments are short, discontinuous DNA segments synthesized on the lagging strand. In E. coli, Okazaki fragments are typically 1,000–2,000 nucleotides long; in eukaryotes, they are shorter, usually 100–200 nucleotides. Each fragment begins with an RNA primer synthesized by primase, which is then extended by DNA polymerase. The number of Okazaki fragments—and therefore the number of primers—depends on the length of the lagging strand being replicated. A single replication fork copying a 1-megabase (Mb) region of the E. coli chromosome would require roughly 500–1,000 primers on the lagging strand, while the leading strand requires only one primer at the origin.
The synthesis of Okazaki fragments is a cyclical process. As the helicase unwinds the duplex, ssDNA on the lagging strand is transiently exposed. Primase binds to this ssDNA, synthesizes a short RNA primer, and then dissociates. DNA polymerase III (in bacteria) or DNA polymerase δ (in eukaryotes) then binds the primer and extends it until it reaches the 5′ end of the preceding Okazaki fragment. The RNA primer is subsequently removed, the gap is filled with DNA, and the fragments are joined by DNA ligase. This cycle repeats many times, making primase one of the most frequently acting enzymes in the replisome.
Mechanism of Primase Action on the Lagging Strand
Primase Binding to Single-Stranded DNA
Primase does not bind to dsDNA; it specifically recognizes and binds to ssDNA. In the replisome, the lagging strand template is coated by single-stranded DNA-binding proteins (SSB in bacteria, RPA in eukaryotes), which must be displaced or accommodated for primase to access the template. Primase has a relatively low affinity for ssDNA compared to SSB, so its recruitment is often facilitated by protein–protein interactions with the replicative helicase. In E. coli, DnaG primase interacts directly with the DnaB helicase, which encircles the lagging strand template. This interaction positions primase near the fork and allows it to capture transient stretches of ssDNA that become available as the helicase unwinds the duplex.
Primase binding to ssDNA is sequence-selective, though the specificity is modest. In E. coli, DnaG recognizes a 3-nucleotide consensus sequence, typically 5′-CTG-3′, on the template strand. The enzyme initiates RNA synthesis at a specific position relative to this sequence, usually 1–2 nucleotides downstream. In eukaryotes, the Pol α-primase complex shows less stringent sequence specificity, though it does exhibit a preference for pyrimidine-rich templates. The binding of primase to ssDNA is also influenced by the length of the available template; a minimum of approximately 10 nucleotides of ssDNA is required for efficient primer synthesis.
RNA Primer Synthesis and Length
Once bound to the template, primase initiates RNA synthesis by catalyzing the formation of a phosphodiester bond between two NTPs. The first nucleotide is typically ATP or GTP, and the second nucleotide is complementary to the template base immediately downstream. The enzyme then processively adds nucleotides in the 5′-to-3′ direction, extending the RNA chain. However, primase is not highly processive; it synthesizes only a short RNA product before dissociating.
The length of the RNA primer varies by organism. In E. coli, DnaG synthesizes primers of 10–12 nucleotides. In eukaryotes, the Pol α-primase complex synthesizes a short RNA primer of approximately 8–10 nucleotides, which is then extended by the DNA polymerase activity of Pol α to produce a total RNA-DNA hybrid primer of about 30 nucleotides. This hybrid primer is subsequently transferred to DNA polymerase δ for processive elongation. The short length of RNA primers is a critical feature: it minimizes the amount of RNA that must later be removed and replaced with DNA, reducing the burden on repair pathways.
Primer synthesis terminates when the primer reaches its characteristic length. The mechanism of termination is not fully understood, but it likely involves a combination of product dissociation, a "counting" mechanism based on the length of the RNA-DNA duplex, and competition from DNA polymerase for the primer terminus. Once the primer is complete, primase releases it, and the primer remains base-paired to the template, presenting a free 3′-OH for DNA polymerase to use.
Handoff to DNA Polymerase
The transfer of the RNA primer from primase to DNA polymerase is a coordinated process that prevents the primer from being lost or degraded. In bacteria, the handoff occurs through a direct protein–protein interaction between DnaG and the β-clamp–DNA polymerase III complex. The β-clamp, a sliding clamp that tethers DNA polymerase to the template, is loaded onto the RNA primer by the clamp loader complex (γ-complex). Once loaded, the β-clamp recruits DNA polymerase III, which displaces primase and begins extending the primer with dNTPs.
In eukaryotes, the handoff is more complex. The Pol α-primase complex synthesizes the RNA primer and then extends it with DNA for about 20 nucleotides, creating an RNA-DNA hybrid primer. This hybrid is then transferred to DNA polymerase δ in a process that requires the sliding clamp PCNA (proliferating cell nuclear antigen) and the clamp loader RFC (replication factor C). The transfer involves a "polymerase switch" in which Pol α dissociates and Pol δ takes over, a process regulated by the differential affinities of the polymerases for PCNA and the primer terminus. This switch is essential for processive lagging strand synthesis, as Pol α is not processive enough to synthesize entire Okazaki fragments.
Primase Structure and Catalytic Activity
Subunits and Domains
The structure of primase reflects its dual roles of template binding and catalysis. In bacteria, DnaG is a single polypeptide with three functional domains: an N-terminal zinc-binding domain (ZBD), a central RNA polymerase domain (RPD), and a C-terminal helicase-binding domain (HBD). The ZBD is involved in sequence-specific recognition of the template and in the initiation of primer synthesis. The RPD contains the active site, where NTPs are bound and phosphodiester bonds are formed. The HBD mediates the interaction with the DnaB helicase, which is essential for primase recruitment to the replication fork.
In eukaryotes, primase is a heterodimer of PRIM1 (p49) and PRIM2 (p58). PRIM1 contains the catalytic active site and is homologous to the RPD of bacterial primase. PRIM2 has a regulatory role: it stabilizes PRIM1, contributes to template binding, and is required for the initiation of primer synthesis. The primase heterodimer is tightly associated with DNA polymerase α (Pol α) through the non-catalytic subunits p70 and p180, forming the Pol α-primase complex. This complex is unique among eukaryotic polymerases in that it can initiate DNA synthesis de novo, a property conferred by the primase subunits.
NTP Selection and Phosphodiester Bond Formation
Primase selects ribonucleoside triphosphates (NTPs) as substrates, not deoxyribonucleoside triphosphates (dNTPs). This selectivity is achieved through the structure of the active site, which contains a tyrosine residue that sterically excludes the 2′-deoxy form of the nucleotide. The active site also contains two metal ions, typically Mg²⁺, which coordinate the triphosphate moiety and the 3′-OH of the growing chain, facilitating the nucleotidyl transfer reaction.
The initiation of primer synthesis is the rate-limiting step and is mechanistically distinct from elongation. During initiation, primase must bring two NTPs together in the active site and catalyze the formation of the first phosphodiester bond. This reaction requires the template to be positioned correctly and the NTPs to be bound in a specific orientation. The initiation step is also the point at which primase exhibits its sequence specificity, as the first two nucleotides must be complementary to the template bases at the initiation site.
After initiation, primase switches to an elongation mode, in which it adds nucleotides processively to the growing RNA chain. The elongation mode is less sequence-specific than initiation, and the enzyme can incorporate any NTP that is complementary to the template. However, the processivity of primase is low, and the enzyme typically dissociates after synthesizing 8–12 nucleotides. The low processivity is thought to be an intrinsic property of the enzyme, determined by the weak binding of the RNA product and the conformational changes that occur upon product release.
Regulation of Primase and Coordination with the Replisome
Interaction with Helicase and Polymerase
Primase activity is tightly regulated to ensure that primers are synthesized at the correct time and place. The primary regulator is the replicative helicase. In E. coli, DnaG binds to DnaB through its C-terminal HBD. This interaction is essential for primase function, as DnaG cannot efficiently locate and bind to ssDNA on its own. The DnaB-DnaG interaction is transient; DnaG binds to DnaB, synthesizes a primer, and then dissociates. The frequency of primer synthesis is controlled by the rate at which DnaG binds to DnaB, which in turn is influenced by the concentration of DnaG and the availability of ssDNA.
In eukaryotes, the interaction between primase and the helicase is mediated by the Cdc45-MCM-GINS (CMG) complex, which is the replicative helicase. The Pol α-primase complex interacts with the CMG complex through the p70 subunit, and this interaction is required for primase recruitment to the fork. The regulation of primase activity in eukaryotes is more complex than in bacteria, involving phosphorylation of primase subunits by cyclin-dependent kinases (CDKs) and other kinases. Phosphorylation of PRIM1 and PRIM2 modulates their activity and their interaction with other replisome components, ensuring that primer synthesis is coordinated with the cell cycle.
Primase Recycling and Primer Counting
The lagging strand requires many primers, and the replisome must recycle primase efficiently. In bacteria, DnaG is not a stable component of the replisome; it binds to DnaB transiently, synthesizes a primer, and then dissociates. The released DnaG can then rebind to DnaB and synthesize another primer. This "hit-and-run" mechanism allows a single primase molecule to synthesize many primers during the course of replication.
The frequency of primer synthesis is not random; it is coordinated with the rate of Okazaki fragment elongation. In E. coli, the timing of primer synthesis is linked to the movement of the replication fork. As the helicase unwinds the duplex, ssDNA on the lagging strand is exposed in a loop, and the length of this loop determines when a new primer is synthesized. The "primer counting" model proposes that primase synthesizes a new primer only after the previous Okazaki fragment has been extended to a certain length. This coordination ensures that the lagging strand is synthesized efficiently without excessive gaps or overlaps.
In eukaryotes, the coordination is even more intricate. The Pol α-primase complex must synthesize a primer, extend it with DNA, and then hand off to Pol δ. This process is regulated by the availability of PCNA, which is loaded onto the primer by RFC. The loading of PCNA triggers the polymerase switch, and the rate of PCNA loading determines the frequency of Okazaki fragment initiation. Additionally, the chromatin remodeling factor FACT (facilitates chromatin transcription) has been implicated in regulating primase activity by promoting the dissociation of Pol α-primase from the template after primer synthesis.
Experimental Methods to Study Primase Function
In Vitro Replication Assays
The study of primase function has been greatly advanced by the development of in vitro replication systems. These systems use purified proteins to reconstitute DNA replication in a test tube, allowing researchers to dissect the contributions of individual components. A typical in vitro replication assay for the lagging strand includes a circular ssDNA template (such as M13 phage DNA), the replicative helicase (DnaB), primase (DnaG), DNA polymerase III holoenzyme, SSB, and the clamp loader complex. The reaction is carried out in a buffer containing 20 mM Tris-HCl (pH 7.5), 10 mM MgCl₂, 1 mM ATP, 100 µM each of dNTPs and NTPs, and 1 mM DTT, at 37°C. The products are analyzed by gel electrophoresis, which reveals the distribution of Okazaki fragments and full-length products.
By omitting or adding specific components, researchers can determine the requirements for primase activity. For example, omitting DnaG from the reaction abolishes lagging strand synthesis, confirming that primase is essential. Adding a mutant DnaG with a defective ZBD can reveal the role of sequence-specific recognition in primer synthesis. These assays have been instrumental in defining the biochemical properties of primase, including its processivity, sequence specificity, and interaction with other replisome components.
Primer Extension Assays
Primer extension assays are used to measure the length and sequence of RNA primers synthesized by primase. In a typical assay, primase is incubated with a radiolabeled ssDNA template and NTPs, and the products are separated by denaturing polyacrylamide gel electrophoresis (PAGE). The radiolabeled products are detected by autoradiography, and their sizes are determined by comparison to a sequencing ladder. This assay reveals the length distribution of primers, which is typically 8–12 nucleotides for bacterial primase and 8–10 nucleotides for eukaryotic primase.
Primer extension assays can also be used to determine the initiation site and sequence specificity of primase. By using templates with defined sequences, researchers can map the exact nucleotides at which primase initiates synthesis. For example, studies with E. coli DnaG have shown that it preferentially initiates at a 5′-CTG-3′ sequence on the template, with the first nucleotide being incorporated 1–2 nucleotides downstream of this motif. These assays have also been used to study the effects of mutations in primase, revealing which residues are critical for catalysis and template recognition.
Mutational Analysis
Mutational analysis is a powerful approach for understanding the structure-function relationships of primase. By introducing specific mutations into the dnaG gene (in bacteria) or the PRIM1 and PRIM2 genes (in eukaryotes), researchers can identify residues that are essential for catalysis, template binding, or interaction with other proteins. For example, mutations in the conserved catalytic residues of DnaG, such as the aspartic acid residues that coordinate the metal ions in the active site, abolish primase activity and are lethal in E. coli. Mutations in the ZBD, such as those that disrupt zinc coordination, impair sequence-specific recognition and reduce the efficiency of primer synthesis.
In eukaryotes, mutational analysis has revealed the importance of the PRIM2 subunit for primase function. Mutations that disrupt the interaction between PRIM1 and PRIM2 abolish primase activity, as do mutations that prevent the association of primase with Pol α. These studies have also identified regulatory phosphorylation sites on PRIM1 and PRIM2, which are targets of CDKs and other kinases. By expressing mutant forms of primase in yeast or human cells, researchers can assess the effects of these mutations on DNA replication and cell viability, providing insights into the physiological roles of primase.
Common Misconceptions and Pitfalls in Understanding Primase
Primase vs. DNA Polymerase
A common error is to confuse primase with DNA polymerase. While both enzymes synthesize nucleic acids, they differ in several fundamental ways. Primase synthesizes RNA, not DNA, and uses NTPs as substrates. DNA polymerase synthesizes DNA and uses dNTPs. Primase can initiate synthesis de novo, whereas DNA polymerase requires a pre-existing 3′-OH. Primase is also much less processive than DNA polymerase, synthesizing only short RNA products before dissociating. Finally, primase does not have proofreading activity, whereas many DNA polymerases do. These differences are not trivial; they reflect the distinct roles of the two enzymes in replication.
RNA Primers are Removed Later
Another misconception is that RNA primers remain in the final DNA product. In fact, RNA primers are transient intermediates that are removed after they have served their purpose. In bacteria, the RNA primers are removed by DNA polymerase I, which has 5′-to-3′ exonuclease activity that degrades the RNA while simultaneously filling the gap with DNA. In eukaryotes, the removal of RNA primers is more complex, involving the endonuclease FEN1 (flap endonuclease 1) and the helicase-nuclease Dna2. The RNA primers are degraded, and the resulting gaps are filled with DNA by DNA polymerase δ. Finally, DNA ligase seals the nick between adjacent Okazaki fragments. The removal of RNA primers is essential for the integrity of the genome, as the presence of RNA in DNA would be recognized as damage and could lead to mutations.
Primase Does Not Require a Primer
A third misconception is that primase, like DNA polymerase, requires a primer to initiate synthesis. This is incorrect. Primase is unique among nucleic acid polymerases in its ability to initiate synthesis de novo. The enzyme can form the first phosphodiester bond between two NTPs without any pre-existing nucleic acid. This property is essential for its function, as it provides the initial 3′-OH that DNA polymerase needs to begin DNA synthesis. The ability to initiate de novo is conferred by the structure of the primase active site, which can bind two NTPs and position them for catalysis without a template-primer duplex.
Summary and Key Takeaways
Primase is an essential enzyme that initiates DNA replication by synthesizing short RNA primers on the lagging strand. Its ability to synthesize RNA de novo provides the 3′-OH groups that DNA polymerases require to begin DNA synthesis. The lagging strand requires multiple primers because it is synthesized discontinuously as Okazaki fragments, each of which is initiated by a new RNA primer. Primase activity is tightly regulated and coordinated with the replisome to ensure efficient and accurate replication. Understanding primase function is critical for understanding DNA replication, and it has implications for human health, as defects in primase are associated with developmental disorders and cancer.
Frequently Asked Questions
Does primase work on the lagging strand?
Yes, primase works on the lagging strand. It synthesizes short RNA primers that initiate the synthesis of Okazaki fragments. The lagging strand is synthesized discontinuously, and each Okazaki fragment requires a new RNA primer. Primase acts repeatedly on the lagging strand, generating the primers needed for each fragment.
Why does primase work on the lagging strand but not the leading strand?
Primase works on the lagging strand because the lagging strand is synthesized discontinuously. The leading strand is synthesized continuously in the same direction as fork movement, requiring only one primer at the origin. The lagging strand, however, is synthesized in the opposite direction, requiring multiple primers to initiate each Okazaki fragment.
What does primase do in DNA replication?
Primase synthesizes short RNA primers that provide the free 3′-OH group required by DNA polymerases to begin DNA synthesis. It initiates replication on both the leading and lagging strands, but it acts repeatedly on the lagging strand to initiate each Okazaki fragment.
Is primase a DNA polymerase?
No, primase is an RNA polymerase. It synthesizes RNA, not DNA, and uses ribonucleoside triphosphates (NTPs) as substrates. DNA polymerases synthesize DNA and use deoxyribonucleoside triphosphates (dNTPs). Primase also differs from DNA polymerase in that it can initiate synthesis de novo without a primer.
How many primers does primase synthesize on the lagging strand?
The number of primers synthesized on the lagging strand depends on the length of the DNA being replicated and the size of the Okazaki fragments. In E. coli, Okazaki fragments are 1,000–2,000 nucleotides long, so a 1-Mb region would require 500–1,000 primers. In eukaryotes, Okazaki fragments are shorter (100–200 nucleotides), so more primers are required per unit length of DNA.
What happens to the RNA primers after DNA synthesis?
RNA primers are removed after they have served their purpose. In bacteria, DNA polymerase I removes the RNA primers and fills the gaps with DNA. In eukaryotes, the endonuclease FEN1 and the helicase-nuclease Dna2 remove the primers, and DNA polymerase δ fills the gaps. DNA ligase then seals the nicks between adjacent Okazaki fragments.
Does primase require a primer to start synthesis?
No, primase does not require a primer. It is unique among nucleic acid polymerases in its ability to initiate synthesis de novo. Primase can form the first phosphodiester bond between two NTPs without any pre-existing nucleic acid, providing the initial 3′-OH that DNA polymerase needs.
Key Takeaways
- Primase is an RNA polymerase that synthesizes short RNA primers de novo, providing the 3′-OH groups required by DNA polymerases to initiate DNA synthesis.
- The lagging strand is synthesized discontinuously as Okazaki fragments, each requiring a new RNA primer synthesized by primase.
- Primase is structurally and mechanistically distinct from DNA polymerase: it uses NTPs, lacks proofreading activity, and can initiate synthesis without a primer.
- Primase activity is regulated by interactions with the replicative helicase and other replisome components, ensuring that primers are synthesized at the correct time and place.
- RNA primers are transient intermediates that are removed and replaced with DNA after Okazaki fragment synthesis is complete.
- Defects in primase function can lead to replication stress, genomic instability, and disease, highlighting the importance of this enzyme in maintaining genome integrity.