Structural biologists at the University of Wisconsin-Madison today published cryo-electron microscopy structures of the complete bacterial DNA replication restart complex — a molecular machine that lets bacteria survive antibiotic attack — and found an unexpected architectural link to the protein that fires up replication from scratch, a discovery that hands drug designers the first atomic-resolution blueprint for disabling a key bacterial survival mechanism.
The paper arrives as antimicrobial resistance kills more than 1.27 million people worldwide each year and forecasts from the Global Research on Antimicrobial Resistance project project 39 million deaths directly caused by resistant infections between 2025 and 2050. Doctors treating drug-resistant infections from E. coli to Neisseria gonorrhoeae rely on antibiotics that work by collapsing bacterial DNA replication. Whether bacteria survive that attack depends on their ability to restart replication — and that restart machinery has, until now, lacked a precise structural map.
Bacteria Use a Dedicated Rescue Machine to Survive Antibiotic-Induced DNA Damage
Most antibiotics in current clinical use, including the widely prescribed fluoroquinolone class — ciprofloxacin, levofloxacin, moxifloxacin — kill bacteria by blocking enzymes that manage DNA topology during replication. Poisoned enzymes act as physical barriers, collapsing the replication fork and leaving the bacterial chromosome half-copied. Bacteria that survive do so because a dedicated rescue system, the primosome, identifies the abandoned fork, processes it, and loads fresh replication machinery back onto the DNA.
Unlike human cells, which can restart replication from thousands of backup origins, most bacteria have a single chromosomal origin and cannot afford to simply begin again elsewhere. Loss of primosome components is lethal, making the pathway essential for bacterial survival — and an attractive target for new drugs or drugs designed to augment existing ones.
Cryo-EM Structures Capture Two Steps in the Assembly Process
In a study posted on bioRxiv in January 2026 and now available on PubMed Central, lead authors Peter L. Ducos and Alexander T. Duckworth, working in the laboratories of James L. Keck and Timothy Grant at the Morgridge Institute for Research and the University of Wisconsin-Madison, used cryo-EM to reconstruct the primosome at near-atomic resolution across two distinct states: an intermediate assembly and a fully mature complex.
The structures show precisely how three proteins — PriA, PriB, and DnaT — assemble in sequence onto an abandoned replication fork. PriA binds first, recognising the fork's branched DNA geometry. PriB docks onto PriA and grips the single-stranded lagging strand. Then DnaT arrives and completes the complex.
The mature structure reveals the mechanism of that final step. Multiple copies of DnaT's C-terminal domain filamented along the single-stranded DNA, physically displacing PriB and handing the strand off in a coordinated transfer. That handoff positions the DNA for loading of DnaB, the helicase that powers the new replisome.
DnaT and DnaA Use the Same Structural Logic — an Unexpected Evolutionary Echo
The most striking result is what the mature structure resembles. DnaA is the master initiator of bacterial DNA replication: it binds the chromosomal origin and melts the double helix to allow the very first replisome to load at the start of a cell cycle. It had been treated as mechanistically separate from the repair-restart pathway.
The new structures show DnaT's C-terminal domain filamenting on single-stranded DNA in a spatial arrangement that closely parallels how DnaA oligomerises around origin DNA, deploying the same structural logic to accomplish what is functionally the same task — opening DNA and presenting it to a helicase loader. As Ducos and colleagues write in the study, the results "suggest an unexpected mechanistic similarity between DnaT and the canonical initiator protein DnaA."
The implication is that replication initiation and replication restart — treated in textbooks as parallel but distinct — may share conserved structural principles, either through evolution maintaining an ancient mechanism or through independent convergence on the same solution.
Drug Designers Now Have Atomic Blueprints Where They Previously Had Sketches
The translational relevance is direct. Fluoroquinolone-based treatments for infections from drug-resistant E. coli and Neisseria gonorrhoeae work partly by collapsing replication forks — but bacteria that can restart replication survive the treatment and contribute to resistance. Blocking replication restart would either kill those bacteria outright or restore sensitivity to antibiotics already in clinical use.
Until this study, the absence of high-resolution structural data for the fully assembled primosome forced drug designers to work from partial structures of individual components. The atomic models now deposited in the RCSB Protein Data Bank change that. Medicinal chemists can now examine the PriA active site and the PriA/PriB and PriB/DnaT interfaces — the points where the step-by-step assembly is most vulnerable — to design small molecules that lock the complex in a non-functional state.
A 2023 review in the International Journal of Molecular Sciences by researchers at James Cook University identified primosomal proteins, including PriA, as "emerging drug targets" in the development of novel antibiotics, noting that DNA replication remains "a largely underexplored" source of new antibacterial compounds compared with how many pathogens have now become resistant to existing drug classes.
A Promising Target but a Long Road to the Clinic
Drug discovery experts caution that a crystal structure — or a cryo-EM structure — is a starting point, not a finished drug. Ajit Parhi, chief scientific officer of TAXIS Pharmaceuticals, which works on novel antibiotic targets, noted in early 2026 that AI and structural data can speed target identification and lead optimisation, but that "many of the predictions remain unverified in animal models or human subjects" — a reminder that structural novelty does not guarantee clinical utility.
The broader landscape is not encouraging for timelines. The WHO's Global Antimicrobial Resistance Surveillance report 2025 found 127 countries now participating in resistance monitoring — but new antibiotic approvals remain rare, and carbapenem-resistant infections in the US surged 69% in recent years, with particularly virulent NDM strains rising by 461%. Any inhibitor of replication restart would need to clear years of safety and efficacy testing before reaching patients.
What This Means for Patients Treated with Antibiotics Today
For the 2.8 million Americans who contract antibiotic-resistant infections each year — and the more than 35,000 who die from them, per CDC data — the primosome structures represent a measurable step toward treatments that either replace failing antibiotics or restore them to effectiveness. The structures also validate the strategy of targeting replication restart as a potentiator of existing drugs.
Clinicians, infection researchers, and pharmaceutical companies can now access the full structural data through the Protein Data Bank and the Electron Microscopy Data Bank. The study itself is freely available as a preprint at bioRxiv, with structural data available for immediate independent analysis and drug screening efforts.
ⓒ 2026 TECHTIMES.com All rights reserved. Do not reproduce without permission.
