The study began with a methodological choice that sets it apart from much of the antibiotic adjuvant literature. Rather than screening libraries of chemicals and then struggling to work out how the hits function, the researchers used CRISPR interference, or CRISPRi, to build a genome-wide gene repression library in Escherichia coli and asked which genes, when silenced, make bacteria more vulnerable to the fluoroquinolone ciprofloxacin. Because CRISPRi represses genes reversibly and without killing the cell, the team could distinguish genes whose loss specifically sensitizes bacteria to antibiotic stress from genes that are simply essential for growth. They exposed the library to ciprofloxacin concentrations ranging from one-eighth to sixteen times the minimum inhibitory concentration, then used high-throughput amplicon sequencing of surviving clones to quantify which guide RNAs had been depleted.

The results pointed unambiguously toward DNA maintenance. Kyoto Encyclopedia of Genes and Genomes pathway analysis showed strong enrichment of sensitivity-related genes in homologous recombination, mismatch repair, and nucleotide excision repair, while Gene Ontology analysis highlighted DNA repair and recombination, the SOS response, membrane organization, and efflux pump regulation. Within the homologous recombination pathway, repression of dnaE, dnaQ, holD, polA, recA, recB, recC, ruvA, and ruvB left bacterial growth essentially untouched in the absence of antibiotics but sharply reduced survival as ciprofloxacin concentrations rose, a pattern the authors describe as synthetic lethality between DNA repair deficiency and antibiotic-induced genotoxic stress. Among these, recA, polA, dnaE, and holD showed the most consistent potentiating effect, not only against quinolones but also against beta-lactams, aminoglycosides, and nitrofurantoin.

Homologous recombination is an attractive target precisely because it is so conserved. Core components such as RecA and RuvABC maintain genome stability, repair DNA damage, and mediate horizontal gene transfer in virtually all bacteria, and their catalytic domains and DNA-binding motifs are nearly identical in organisms as different as E. coli and Bacillus subtilis. To validate the screen, the team deleted several nonessential genes, including recA, polA, holD, ruvB, and dnaQ. Although minimum inhibitory concentrations were unchanged, all five deletion strains showed significantly reduced survival after exposure to one or more bactericidal antibiotics, and recA deletion reduced survival under every bactericidal drug tested while leaving susceptibility to bacteriostatic agents such as tetracycline and tigecycline intact.

The mechanistic story that emerged was broader than simple loss of DNA repair. Compared with the wild-type parent strain, the recA-deficient bacteria displayed significant membrane damage, with increased permeability and fluidity and a reduced membrane potential, all of which promote antibiotic influx. Ethidium bromide accumulation assays revealed impaired efflux pump function, and enzyme-linked immunosorbent measurements confirmed heightened intracellular accumulation of ciprofloxacin. The mutants also produced elevated levels of reactive oxygen species, showed attenuated antioxidant capacity, and suffered pronounced declines in ATP production, respiratory rate, and the transmembrane proton gradient, with swimming motility compromised as well. The authors argue that these phenotypes stem not only from the loss of RecA’s recombinase activity but also from secondary disruption of the SOS response and broader transcriptional dysregulation, effectively turning RecA into a global regulator of bacterial stress physiology.

Perhaps the most striking discovery was that RecA also facilitates the horizontal spread of resistance. Because ATP production, respiration, and motility all regulate plasmid conjugation, the team tested whether recA deficiency affects plasmid transfer. Transconjugation experiments with the RP4-7 plasmid showed that recA-deficient strains, whether serving as donors or recipients, transferred the plasmid at significantly reduced frequency, with the defect most severe when both partners lacked RecA. A target that simultaneously sensitizes bacteria to antibiotics and blocks the dissemination of resistance genes is, in principle, a doubly valuable asset for anti-resistance strategies.

With the target validated genetically, the researchers tested three ways of inhibiting it. First, they delivered a CRISPRi system targeting recA by bacterial conjugation; transcriptional knockdown was successful and sensitized cells to ciprofloxacin and meropenem, but the effect was modest, reflecting the practical limits of plasmid-based delivery. Second, they exploited RecX, a natural RecA inhibitor that suppresses RecA-mediated strand exchange even at substoichiometric levels. Overexpression of recX reduced survival under quinolone treatment in a dose-dependent manner, and a synthetic twenty-amino-acid alpha-helical peptide derived from the RecX-RecA structure, fused to a cell-penetrating motif, potentiated quinolone killing, though its activity did not extend to beta-lactams, aminoglycosides, or colistin.

The third strategy proved the most powerful. Computational docking of a library of 135 candidate compounds against RecA identified cisplatin, the well-known platinum-based anticancer drug, as the strongest potentiator. Cisplatin significantly enhanced ciprofloxacin’s bactericidal effect against wild-type E. coli but showed no synergy in the recA deletion strain, demonstrating that the interaction is RecA-dependent. Surface plasmon resonance confirmed direct, concentration-dependent binding of cisplatin to RecA between 25 and 400 micromolar, with a dissociation constant of 40.3 micromolar, and comet assays revealed DNA fragmentation in cisplatin-treated cells resembling that of recA-deficient mutants. Time-kill assays showed a three-log reduction in viability within twenty-four hours at 8 micrograms per milliliter of cisplatin, with survival falling below the detection limit at 16 micrograms per milliliter after forty-eight hours. The synergy held against twenty multidrug-resistant clinical E. coli isolates and extended to quinolones, beta-lactams, aminoglycosides, polymyxins, and rifampicin in E. coli, Klebsiella pneumoniae, Proteus mirabilis, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant Enterococcus faecalis. Other platinum drugs, including lobaplatin, nedaplatin, carboplatin, and oxaliplatin, also potentiated antibiotics, though more weakly. Critically, serial passaging showed that cisplatin substantially delayed the emergence of ciprofloxacin resistance.

Cisplatin also suppressed conjugative transfer of clinically critical resistance plasmids, including those carrying tet(X4), bla NDM-5, and mcr-1, across a wide range of temperatures, pH values, plasmid incompatibility types, and donor-recipient combinations spanning E. coli, Salmonella Typhimurium, and K. pneumoniae. In animal models, the combination therapy achieved a seventy-five percent survival rate in Galleria mellonella larvae infected with a multidrug-resistant isolate, significantly outperforming either monotherapy, and reduced bacterial loads in the colon, liver, spleen, lung, and kidney in mouse intestinal and peritonitis-sepsis models while lowering pro-inflammatory cytokines and raising anti-inflammatory ones. In vivo transconjugation assays showed that cisplatin curtailed plasmid transfer in the mouse intestine within forty-eight hours without disturbing bacterial colonization. Safety testing was encouraging: hemolysis remained below five percent at 160 micrograms per milliliter, twenty times the synergistic dose, HEK293T cell viability exceeded ninety-five percent at the same concentration, and histopathology of five major organs in treated mice appeared normal, though the authors caution that cisplatin’s known toxicity and off-target effects will require structural optimization and drug-drug interaction studies before clinical translation.

The work frames what the authors call a defense-targeting adjuvant concept: rather than attacking essential bacterial structures directly, adjuvants disable the resilience machinery that pathogens rely on to survive antibiotic stress. By identifying homologous recombination, and RecA in particular, as a conserved, broadly applicable vulnerability, the study offers a target-driven blueprint for discovering next-generation potentiators and a feasible path toward restoring the efficacy of the antibiotic arsenal already in hand.

Subject of Research: Targeting the bacterial homologous recombination pathway and RecA protein to develop broad-spectrum antibiotic adjuvants against multidrug-resistant pathogens

Article Title: A genome-wide CRISPRi screen identifies homologous recombination pathway as potential target for broad-spectrum antibiotic adjuvants

Article References: Zhang, H., Chen, B., Gu, L., Wang, C., Xu, L., Ji, X., Wang, J., Wang, Z., Xiao, X., & Liu, Y. (2026). A genome-wide CRISPRi screen identifies homologous recombination pathway as potential target for broad-spectrum antibiotic adjuvants. Journal of Advanced Research, 87, 947-962. https://doi.org/10.1016/j.jare.2025.12.015

Image Credits: AI Generated

DOI: 10.1016/j.jare.2025.12.015

Keywords: antibiotic resistance, antibiotic adjuvants, CRISPRi screen, homologous recombination, RecA, cisplatin, horizontal gene transfer, multidrug-resistant bacteria, DNA repair, SOS response, conjugation, antimicrobial potentiators

Checkpoint kinase 2, encoded by the CHEK2 gene, sits at a pivotal node in the DNA damage response. When breaks are detected, the master transducer ATM phosphorylates checkpoint kinase 2, which in turn propagates the alarm by phosphorylating a panel of downstream targets, including the tumor suppressor p53, the checkpoint regulator BRCA1 and the cell-cycle effector CDC25A. The result is a coordinated halt in cell division that buys time for repair, or, if the damage is beyond salvation, steers the cell toward senescence or apoptosis. Because biallelic loss-of-function mutations in CHEK2 confer a substantially elevated risk of breast cancer and other malignancies, understanding precisely what the kinase does, and what cells do without it, has occupied genome stability researchers for more than two decades.

The central question addressed in the new work is one of pathway choice. Mammalian cells deploy two principal strategies to mend double-strand breaks. Homologous recombination is the high-fidelity route: it uses the intact sister chromatid as a template and is largely restricted to the S and G2 phases of the cell cycle, when such a template exists. Non-homologous end joining, by contrast, can operate throughout the cell cycle. It directly religates broken ends, quickly but with the potential for small insertions or deletions at the junction. A third pathway, alternative end joining or microhomology-mediated end joining, relies on short exposed sequence repeats and is generally considered more error-prone still. Which pathway a cell engages for any given break has profound consequences: homologous recombination preserves the genetic message, while the end-joining routes can quietly rewrite it.

Pathway choice is not random. It is orchestrated by a molecular choreography that begins with the rapid accumulation of the MRE11-RAD50-NBS1 complex and the signaling protein 53BP1 at break sites. A tug-of-war then ensues. 53BP1, together with its effectors RIF1 and the shieldin complex, blocks the nucleolytic resection of DNA ends, thereby favoring end joining. BRCA1, in combination with PALB2 and BRCA2, promotes the removal of 53BP1 and supports the long-range resection that generates the single-stranded DNA overhangs required for homologous recombination. Cell-cycle cues, chromatin state and the availability of key enzymes all tilt this balance. Checkpoint kinase 2 has long been suspected of influencing the process, both through its well-characterized phosphorylation of BRCA1 and through its role in enforcing the cell-cycle checkpoints that determine whether a sister chromatid template is even available.

According to the study, checkpoint kinase 2 deficiency measurably affects how cells engage these repair pathways after DNA damage. Rather than a simple loss of repair capacity, the deficiency appears to shift the relative engagement of the competing routes, altering the balance between resection-dependent, template-directed repair and direct end joining. This distinction matters because a cell can maintain apparently adequate bulk repair throughput while quietly accumulating a different spectrum of errors. The work suggests that the kinase functions not merely as an amplifier of the damage signal but as a determinant of repair-pathway engagement, embedding cell-cycle and damage-load information into the repair decision itself.

The experimental logic behind such conclusions typically rests on a combination of genetic manipulation and reporter assays. Researchers induce defined double-strand breaks with site-specific nucleases or ionizing radiation, then measure the relative use of homologous recombination and end joining with engineered fluorescent or antibiotic-resistance reporters in which restoration of a disrupted gene depends on a specific repair route. Complementary biochemical readouts, including chromatin immunoprecipitation for repair factors such as RAD51, 53BP1 and RIF1, and assays of single-stranded DNA generation at break sites, reveal how the recruitment landscape changes when checkpoint kinase 2 is absent. Cell-cycle fractionation is essential, since the phases in which homologous recombination is available are exactly the phases most affected by checkpoint loss, and the new study’s emphasis on engagement rather than raw capacity points to analyses of this kind.

Why should a signaling kinase have a hand in pathway choice at all? One likely answer lies in the temporal logic of the DNA damage response. Checkpoint kinase 2 activation is among the earliest events after a break occurs, and its phosphorylation of CDC25A triggers the degradation of that phosphatase, preventing cells from entering or progressing through S phase while breaks persist. This checkpoint function is intimately tied to resection biology: productive homologous recombination requires time, a sister chromatid and a permissive cell-cycle window, all of which are guaranteed by an intact checkpoint. Without checkpoint kinase 2, cells may proceed into or through S phase with unrepaired breaks, encounter lesions without an appropriate template context, and default more heavily toward end-joining mechanisms that demand no such coordination. The kinase’s phosphorylation of BRCA1, meanwhile, has been implicated in recruiting and stabilizing the recombination machinery at damage sites, providing a second, more direct link to pathway selection.

The clinical resonance of these findings is difficult to overstate. CHEK2 is one of the most frequently mutated moderate-risk breast cancer susceptibility genes identified to date, carried by a meaningful fraction of women in population cohorts across Europe and North America. If loss of the kinase biases cells toward error-prone repair in specific contexts, that bias could help explain why CHEK2 carriers accumulate oncogenic mutations over a lifetime, and why their tumors display characteristic patterns of genomic scarring. There is also a therapeutic dimension. Inhibitors of poly(ADP-ribose) polymerase exploit the dependence of BRCA-deficient tumors on alternative repair routes, and a refined understanding of how checkpoint kinase 2 loss reshapes pathway engagement could inform whether CHEK2 mutation carriers respond differently to PARP inhibitors, radiation or certain chemotherapeutics that inflict DNA damage deliberately.

The study also speaks to a broader conceptual shift in genome biology. For many years, the DNA damage response was portrayed as a linear circuit: damage in, signal transduced, repair out. The contemporary picture is far more networked, with feedback loops, phase-specific constraints and kinetic competition among repair factors deciding the fate of each lesion. Checkpoint kinases were initially assigned narrow roles as clock-setters, pausing the cycle while repair proceeded. The accumulating evidence, including the pathway-engagement effects documented in this study, suggests instead that signaling and repair are intertwined at the level of mechanism, not merely sequence. The kinase does not simply buy time for repair; it helps determine which repair will occur.

Open questions remain. The precise phosphorylation events that link checkpoint kinase 2 to the resection machinery are still being mapped, and the extent to which the pathway-choice effects seen in cell models generalize to human tissues bearing heterozygous CHEK2 mutations, the situation in most carriers, awaits further investigation. It will also be important to determine whether the altered repair balance in checkpoint kinase 2-deficient cells produces the mutation signatures now detectable in tumor genomes, allowing epidemiologists to connect carrier status to specific patterns of somatic evolution. What the study establishes is that a deficiency in checkpoint kinase 2 changes not just the speed of the cellular response to double-strand breaks but its character, tilting the molecular tug-of-war that decides whether the genome’s severed strands are stitched back together faithfully or patched in ways that leave a permanent, and potentially dangerous, record. In the ongoing effort to understand why some inherited variants so potently predispose to cancer, that shift in repair engagement may prove to be one of the most consequential consequences of losing this guardian of the genome.

The study’s timing is notable given renewed interest in checkpoint kinases as drug targets. Selective checkpoint kinase 2 inhibitors have been explored in oncology, partly on the premise that transient checkpoint loss can sensitize tumors to DNA-damaging agents by forcing cells to divide before repair is complete. The observation that the kinase influences which repair route is engaged adds a further consideration: pharmacological inhibition might not simply accelerate breakage-driven death in cancer cells but could also reshape repair choices in exposed normal tissue, a variable worth measuring in preclinical safety work.

The findings may also intersect with tissue-specific mutation patterns seen in CHEK2 families. Unlike BRCA1 and BRCA2, which confer pronounced ovarian cancer risk, CHEK2 mutations are associated predominantly with breast cancer, with weaker or uncertain links to other tumor types. A repair-pathway explanation would predict that the consequences of losing the kinase depend on how often a given tissue relies on the routes whose engagement is altered, offering a framework for those epidemiological differences.

Methodologically, distinguishing a genuine shift in pathway engagement from a secondary consequence of checkpoint failure remains analytically demanding. Because checkpoint loss changes cell-cycle distributions, apparent differences in reporter outcomes can reflect altered timing rather than altered mechanism, making properly controlled, phase-matched comparisons essential for interpreting this and future studies of signaling kinases in repair decisions.

Subject of Research: The role of checkpoint kinase 2 in DNA double-strand break repair pathway choice

Article Title: Checkpoint kinase 2 deficiency affects the engagement of DNA double-strand break repair pathways following DNA damage

Article References: Muñoz-Maldonado, C., Etter, R., Quintin, A., Degen, P. M., Medo, M., Aebersold, D. M., Zimmer, Y., & Medová, M. (2026). Checkpoint kinase 2 deficiency affects the engagement of DNA double-strand break repair pathways following DNA damage. Cell Death Discovery. https://doi.org/10.1038/s41420-026-03340-3

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03340-3

Keywords: checkpoint kinase 2, CHEK2, DNA double-strand breaks, homologous recombination, non-homologous end joining, DNA damage response, BRCA1, ATM, genome stability, cancer susceptibility, pathway choice, DNA repair

The review, published on 26 August 2026 by Xinyan Zou, Botao Zhang, Binxian Gu, Yong Gong, Yanchao Bai and Likui Zhang, focuses on NurA as a central component of a compact DNA end-resection machine. In archaeal cells, NurA works together with the HerA helicase and the Mre11-Rad50 complex. Their combined task is to process the broken ends of DNA and generate single-stranded DNA tails with a free 3′ end. These tails are essential substrates for homologous recombination, a repair pathway that uses an intact DNA molecule as a template to restore missing genetic information.

The first challenge after a double-strand break is not simply joining the broken ends. For homologous recombination to begin, the cell must carefully reshape the damaged DNA. One strand is progressively removed from each broken end, exposing the complementary strand as single-stranded DNA. This directional process, known as end resection, creates a molecular platform on which recombination proteins can assemble. The resulting 3′ single-stranded tails are particularly important because they can search for and pair with a matching sequence in an undamaged chromosome. NurA appears to perform a major part of the enzymatic cutting required to produce these tails, while HerA helps unwind the DNA and Mre11-Rad50 contributes to the initial recognition and processing of the break.

Structurally, archaeal NurA forms a toroidal dimer, producing a ring-shaped molecular assembly with a central channel. This architecture is more than a visual feature. A ring-shaped nuclease can help confine DNA within a defined catalytic environment, positioning the nucleic acid for controlled cleavage as it passes through or interacts with the channel. The dimeric arrangement also creates an extended surface for DNA binding and may help coordinate the movement of DNA with the action of the associated HerA helicase. According to the review, NurA and HerA can form a continuous channel, suggesting that DNA may be transferred directly from the helicase into the nuclease without freely diffusing into the surrounding cell.

NurA possesses two related but distinct catalytic capabilities. It can act as a 5′-to-3′ exonuclease, removing nucleotides sequentially from a DNA end, and it can also function as an endonuclease, cutting within a DNA strand rather than only at its terminus. These activities provide the enzyme with flexibility during repair. Exonucleolytic digestion can enlarge a resected region from an existing DNA end, while endonucleolytic cleavage may help initiate processing at particular DNA structures or positions. The balance between the two activities is likely to depend on the DNA substrate, the arrangement of the protein complex and the presence of partner enzymes, although the review emphasizes that important mechanistic questions remain unresolved.

The chemical engine behind NurA’s activity is a divalent metal ion. In particular, manganese ions, or Mn²⁺, are described as essential for catalysis. Like many nucleases, NurA is thought to use metal ions to organize the DNA substrate, activate water molecules and stabilize the negatively charged reaction intermediates that form when phosphodiester bonds are broken. The requirement for Mn²⁺ highlights the importance of the enzyme’s catalytic environment: without the appropriate metal cofactor, the chemical steps needed to cleave DNA cannot proceed efficiently. The review also summarizes the identification of amino acids that contribute to DNA binding and strand cleavage, helping define how NurA recognizes its substrate and positions the scissile phosphate bond for hydrolysis.

The NurA–HerA partnership is particularly significant because it links two physically different operations: DNA unwinding and DNA degradation. HerA is a helicase that uses energy from nucleotide hydrolysis to separate the two strands of the DNA duplex. NurA, positioned alongside it, can then process the exposed strand. Their continuous channel offers a possible explanation for how archaeal cells coordinate these reactions while minimizing the risk of uncontrolled DNA degradation. Rather than allowing a nuclease and helicase to operate independently, the paired system could guide DNA through a protected molecular corridor in which unwinding and resection are synchronized. The Mre11-Rad50 complex adds another layer of coordination by participating in the detection and early processing of broken chromosome ends.

This compact repair system is also important from an evolutionary perspective. NurA homologs are found across archaeal lineages and in some bacteria, but no NurA counterpart is known in eukaryotes. At the same time, eukaryotic cells possess more elaborate DNA end-resection pathways that perform related functions through different protein assemblies. The presence of NurA in archaea, together with its partnership with HerA and Mre11-Rad50, raises questions about how ancient DNA repair strategies were reorganized during the transition from prokaryotic ancestors to the first eukaryotic cells. The review presents NurA as a potentially valuable molecular marker for tracing which parts of double-strand break repair machinery were retained, replaced or lost during evolution.

Particular attention is drawn to the Asgard superphylum, a group of archaea considered especially relevant to studies of eukaryotic origins. Genomic surveys have identified widespread, previously uncharacterized NurA homologs in Asgard archaea. These proteins could reveal whether the structural and catalytic features observed in other archaeal NurA enzymes were already present in lineages related to the ancestors of eukaryotes. However, the review makes clear that Asgard-derived NurA proteins have not yet been experimentally characterized. Their biochemical activities, metal requirements, oligomeric states and interactions with HerA or Mre11-Rad50 remain open questions. Sequence similarity alone cannot establish whether these proteins possess the same dual nuclease activities or assemble into comparable DNA-processing channels.

The authors therefore identify structural biology and biochemistry as priorities for the next stage of research. Determining the three-dimensional structures of Asgard NurA proteins, ideally in complexes with DNA, HerA and metal ions, could show whether their active sites and central channels resemble those of previously studied archaeal enzymes. Complementary experiments could measure their exonuclease and endonuclease activities, define their substrate preferences and test how mutations in DNA-binding or catalytic residues affect repair-related reactions. Reconstituting the NurA–HerA–Mre11-Rad50 system outside the cell would be especially informative because it could reveal how each component contributes to end resection and whether the proteins form a stable, coordinated machine. By connecting molecular structure with biochemical function and evolutionary distribution, NurA research may help explain how cells across the domains of life learned to repair broken chromosomes—and why some lineages ultimately replaced this ancient strategy with entirely different molecular solutions.

Subject of Research: Archaeal NurA nuclease and its role in DNA double-strand break repair

Article Title: Archaeal NurA nuclease: structure, catalytic mechanism, and functional cooperation in DNA double-strand break repair

Article References: Zou, X., Zhang, B., Gu, B. et al. “Archaeal NurA nuclease: structure, catalytic mechanism, and functional cooperation in DNA double-strand break repair.” Applied Microbiology and Biotechnology (2026). https://doi.org/10.1007/s00253-026-14009-3

Image Credits: AI Generated

DOI: https://doi.org/10.1007/s00253-026-14009-3

Keywords: NurA nuclease; Archaea; DNA double-strand break repair; HerA helicase; Mre11-Rad50; homologous recombination; DNA end resection; Mn²⁺-dependent nuclease; Asgard archaea; molecular evolution