DnaE uses strand displacement synthesis during Okazaki fragment repair

This study reveals that in *Bacillus subtilis*, the replicative polymerase DnaE compensates for the absence of DNA polymerase I by utilizing strand displacement synthesis to efficiently repair Okazaki fragments, demonstrating the bacterium's ability to employ multiple mechanisms to maintain lagging strand replication fidelity.

The Gist

Imagine your DNA as a massive, intricate instruction manual for building a living cell. Every time a cell divides, it needs to make a perfect photocopy of this manual. However, there's a catch: the machine that reads the manual (the replication fork) can only read in one direction.

To get around this, the cell builds the "leading strand" (the easy part) in one smooth, continuous line. But the "lagging strand" (the tricky part) has to be built in tiny, disconnected chunks called Okazaki fragments. Think of these like laying down a brick wall where you can only lay one brick at a time, then have to jump back to start the next one.

The Problem: The "Sticky Notes"

To start each of these tiny bricks, the cell uses a temporary "starter" made of RNA. In our analogy, imagine these starters are sticky notes placed on the wall to tell the bricklayer where to begin. Once the brick (DNA) is laid, the sticky note (RNA) must be removed and replaced with a real brick so the wall is solid and continuous.

In most bacteria (like E. coli), there is a specialized "cleanup crew" called DNA Polymerase I (Pol I). Its job is to rip off the sticky note and fill in the gap with a real brick. If you remove this crew in E. coli, the wall falls apart, and the bacteria die.

The Surprise: Bacillus subtilis is Different

The researchers in this paper studied a different bacterium, Bacillus subtilis (B. subtilis). They expected that if they removed the cleanup crew (Pol I), the bacteria would die or struggle.

But they didn't. The bacteria grew just fine! They were almost identical to the normal bacteria. This was a huge mystery. If the main cleanup crew is gone, who is doing the work?

The Investigation: Who is the Substitute?

The scientists decided to play detective in a test tube (in vitro) to see which other proteins could step in. They looked at the two main "bricklayers" (polymerases) that build the DNA wall: PolC and DnaE.

Here is what they discovered, using some fun analogies:

1. The "Bricklayer" (PolC) is Rigid

PolC is the main worker that builds the long stretches of the wall. However, it's very rigid. If it hits a sticky note (RNA) or a gap, it just stops. It can't push the sticky note out of the way. It needs a helper to clear the path first. Without help, it's useless for fixing these gaps.

2. The "Swiss Army Knife" (DnaE) is the Hero

The scientists found that DnaE is the real hero here.

• The Analogy: Imagine DnaE is a construction worker with a powerful crowbar.
• The Action: When DnaE encounters a gap with a sticky note (RNA) blocking the way, it doesn't stop. Instead, it uses its "crowbar" ability (called strand displacement synthesis) to physically push the sticky note out of the way while simultaneously laying down new bricks behind it.
• The Result: It creates a flap of the old sticky note, which can then be easily snipped off by another tool called FEN (a pair of molecular scissors).

This means that in B. subtilis, if the main cleanup crew (Pol I) is missing, DnaE steps up, uses its crowbar to push the RNA out of the way, and finishes the job.

The "Backup Plan"

The study also found a backup plan. If the "crowbar" (DnaE) isn't there, the rigid worker (PolC) can still do the job, but only if the scissors (FEN) or another helper (RNase HIII) cuts the sticky note off before PolC arrives. It's a slower, less efficient process, but it works in a pinch.

Why Does This Matter?

1. Bacteria are Smarter Than We Thought: We used to think there was only one way to fix these DNA gaps. This paper shows that B. subtilis has multiple backup plans, making it very resilient.
2. Evolutionary Cousins: This mechanism (using a "crowbar" to push RNA out) is actually very similar to how human cells fix their DNA. This suggests that even though bacteria and humans are very different, they share some ancient, clever tricks for building life.
3. New Drug Targets: Since DnaE is essential for this backup repair, scientists might be able to design drugs that jam DnaE's "crowbar." If you stop the bacteria from fixing its DNA when the main crew is busy, the bacteria might die. This could lead to new antibiotics.

The Bottom Line

In the world of Bacillus subtilis, if the main cleanup crew (Pol I) is on vacation, the worker DnaE doesn't just sit around. It grabs a crowbar, pushes the temporary sticky notes out of the way, and finishes building the wall. This clever backup system ensures the bacteria's DNA stays intact, even when things go wrong.

Technical Summary

1. Problem Statement

• Context: DNA replication requires the removal of RNA primers from Okazaki fragments on the lagging strand to ensure genome stability. In Escherichia coli, DNA Polymerase I (Pol I) is the primary enzyme responsible for removing these primers via its 5'→3' exonuclease activity and replacing them with DNA via strand displacement synthesis.
• The Anomaly: While E. coli $\Delta polA$ mutants are severely compromised, Bacillus subtilis $\Delta polA$ mutants exhibit a near wild-type phenotype with only minor growth defects. This suggests that B. subtilis possesses alternative mechanisms for Okazaki fragment maturation that do not rely on Pol I.
• The Gap: The identity of the polymerase(s) compensating for the loss of Pol I in B. subtilis and the specific molecular mechanisms they employ (particularly regarding strand displacement synthesis) were previously unknown.

2. Methodology

The authors employed a combination of in vivo phenotypic analysis and in vitro biochemical assays:

• Genetic & Phenotypic Analysis:

  • Generated single and double deletion mutants in B. subtilis (targeting polA, rnhC [RNase HIII], and fenA [FEN]).
  • Spot Titer Assays: Tested sensitivity to DNA damaging agents (Hydroxyurea, Mitomycin C, MMS, Ciprofloxacin) to assess replication stress.
  • SOS Response Assay: Used a dinC-gfp reporter fusion to measure DNA damage response induction via fluorescence microscopy.
  • Microscopy: Visualized cell morphology, nucleoid segregation (using DAPI), and membrane integrity (using FM4-64) to detect anucleate cells, guillotined chromosomes, and filamentation.
  • Fluorescence Reporter: Measured dnaE expression levels using a DnaE-mCitrine fusion in various mutant backgrounds.

• In Vitro Biochemical Assays:

  • Protein Purification: Purified B. subtilis Pol I, PolC, DnaE, RNase HIII, and FEN.
  • Substrate Design: Created synthetic oligonucleotide substrates mimicking Okazaki fragments:
    • Primer-only: No downstream fragment.
    • Nicked: Downstream fragment annealed directly to the primer.
    • Gapped: A 10-nucleotide gap between the primer and the downstream fragment.
    • Flapped: A downstream fragment with a 5' overhang.
  • Extension Assays: Incubated polymerases with substrates and dNTPs, with or without pre-incubation of nucleases (RNase HIII or FEN).
  • Analysis: Visualized products via Urea-PAGE to distinguish between full-length extension, partial extension, and degradation products.

3. Key Contributions

• Identification of DnaE as a Compensatory Polymerase: Demonstrated that the replicative polymerase DnaE, typically known for extending RNA primers, can perform robust strand displacement synthesis to replace Pol I in Okazaki fragment maturation.
• Mechanistic Distinction between PolC and DnaE: Showed that PolC (the main replicative polymerase in B. subtilis) is incapable of strand displacement synthesis on annealed substrates but can function in repair if the downstream fragment is degraded by a nuclease (FEN) or if a flap exists.
• Proposed Alternative Pathways: Defined two novel pathways for Okazaki fragment repair in the absence of Pol I, contrasting with the canonical Pol I-dependent pathways.
• Regulatory Insight: Revealed that dnaE expression is upregulated when lagging strand repair proteins (Pol I, FEN, RNase HIII) are absent, suggesting a transcriptional compensation mechanism.

4. Key Results

• Phenotypic Severity: Single deletions ($\Delta polA$, $\Delta fenA$, $\Delta rnhC$) showed mild sensitivity to DNA damage and minimal SOS induction. However, double deletions ($\Delta polA \Delta rnhC$ and $\Delta fenA \Delta rnhC$) resulted in severe growth defects, hypersensitivity to DNA damage, significant SOS induction (up to 24-fold), and severe chromosome segregation defects (anucleate cells, guillotined nucleoids). This confirms that B. subtilis relies on redundant pathways for lagging strand repair.
• DnaE Strand Displacement:
  • On a gapped substrate (mimicking in vivo conditions), DnaE efficiently performed strand displacement synthesis, generating full-length products similar to Pol I.
  • PolC arrested at the 5' end of the downstream fragment on gapped substrates and could not displace the strand.
  • DnaE activity was further enhanced by the presence of FEN, which cleaves the displaced RNA flap.
• PolC Limitations and Rescue:
  • PolC failed to extend on gapped substrates alone.
  • PolC could generate full-length products only when FEN was present to degrade the downstream fragment before the polymerase reached it, or when a 5' flap was already present.
• Substrate Specificity: DnaE's strand displacement activity was observed on both RNA-DNA hybrid substrates and DNA-only substrates, suggesting a broader role in DNA repair and lesion bypass.
• Expression Upregulation: Fluorescence reporter assays showed that DnaE levels significantly increase in $\Delta polA$, $\Delta fenA$, and $\Delta rnhC$ backgrounds, indicating the cell actively upregulates DnaE to compensate for repair deficiencies.

5. Significance

• Redefining Lagging Strand Maturation: The study overturns the dogma that Pol I is the sole or primary enzyme for Okazaki fragment maturation in B. subtilis. It establishes a model where DnaE acts as a "backup" polymerase capable of strand displacement, working in concert with FEN.
• Evolutionary Insight: The proposed mechanism (DnaE extending and displacing, followed by FEN cleavage) bears a striking resemblance to the eukaryotic Okazaki fragment maturation pathway (Pol $\alpha$/Pol $\delta$ and FEN1), suggesting convergent evolution or a shared ancestral mechanism distinct from the E. coli model.
• Therapeutic Implications: Since DnaE and PolC are C-family polymerases unique to bacteria, understanding their specific roles in replication and repair highlights them as potential targets for novel antibiotics. Specifically, inhibiting DnaE could cripple the backup repair pathway in B. subtilis and related Gram-positive pathogens, potentially sensitizing them to DNA damage.
• Genome Stability: The work elucidates how bacteria maintain genome integrity despite the loss of canonical repair enzymes, emphasizing the redundancy and plasticity of the replisome.