Sommerova et al. show that ataxia telangiectasia-mutated (ATM) protects replicating cells by restraining pathological bypass of endogenous oxidative base lesions through PRIMPOL-mediated repriming. When ATM is lost, post-replicative adducted ssDNA gaps accumulate, creating dependence on homologous recombination for repair and driving PARP hyperactivation and PARP inhibitor sensitivity.

The human genome encodes several hundreds of ubiquitin E3 enzymes, most of which have poorly understood biological roles. In search for novel ubiquitin E3 enzymes involved in DNA damage tolerance, we identified RNF25 as a candidate to have a role in DNA replication stress tolerance. Under stress conditions, RNF25 translocates to the nucleus in a cGAS-dependent manner, and loss of RNF25 ubiquitin E3 activity leads to the accumulation of replication-dependent ssDNA gaps. Combining functional assays with with mass-spectrometry based proteomics approaches such as TULIP2, iPOND-MS and TurboID, we found that, mechanistically, RNF25 promotes the stability of the replication fork at Transcription-Replication Conflicts by the ubiquitin-mediated clearance of RAD18, mono-ubiquitinated PCNA, H2B-K120ub and RECQL, among others. Lack of RNF25 ubiquitin E3 activity promotes the occurrence of Transcription-Replication Conflicts and destabilizes reversed replication forks, enabling re-priming and accumulation of ssDNA gaps behind the replication fork, ultimately compromising genome integrity. Overall, RNF25 regulates DNA replication, transcription and translation in an ubiquitin-based goldilocks.

In this issue of Molecular Cell, Koo et al. describe how the yeast Rad55-Rad57-SHU complex dynamically nucleates Rad51 filaments on DNA. Functional coupling between Rad55-Rad57 and Rad51 on ssDNA seeds filament growth, whilst ATP hydrolysis promotes complex turnover.

Using X-ray crystallography, biochemistry and HDX-MS, this study sheds light on how ssDNA allosterically modulates the activity of DriD, a Caulobacter crescentus transcription activator involved in a non-canonical DNA damage response pathway.

Single stranded RNA (ssRNA) and single stranded DNA (ssDNA) bacteriophages represent a key component of the global virome, yet the host genetic networks supporting their infection cycles remain poorly understood. Here, we present a comprehensive, genome-wide mapping of the genetic landscape regulating infection cycles for F pilus-dependent ssRNA and ssDNA phages in Escherichia coli. Genetic screens across ssRNA phages spanning all four genogroups of the Leviviricetes revealed a highly conserved network of host dependencies, with the notable exception of the F plasmid gene traD. While primary structural receptor components and dsbA mediated disulfide bond formation are universally required across all lineages to ensure F pilus integrity, traD exhibits a strict genogroup-specific requirement during entry, showing variable essentiality across different viral groups despite sharing an identical primary receptor. Our gene dosage screens revealed that an elevated copy number of the hslU protease or the RNA chaperone stpA restricts infection, identifying clear genetic barriers that can perturb the viral life cycle. Parallel assays with filamentous ssDNA phages produced host factor profiles consistent with published literature, while revealing additional variations in host dependency. These screens confirmed that ssDNA phages strictly rely on the host TolQRA complex for entry downstream of pilus engagement. The assays tracked prominent negative fitness signatures across homeostatic clusters, highlighting how the physiological burden of continuous, non-lytic virion extrusion strains the host envelope. Finally, this comparative approach traced the selectivity of our isolation host (E. coli HSF) to a horizontally acquired capsule architecture from Klebsiella. This surface shield excludes a large panel of double stranded DNA phages isolated on diverse E. coli strains, while allowing virions from ssDNA and ssRNA phages to engage the extended F pilus and bypass the barrier via native pilus retraction. Together, this work provides a systematic, class-wide map of single stranded phage-host interactions, bridging classical genetics with modern viral discovery while establishing a robust host platform to access uncultured viral diversity and a functional blueprint to design next generation diagnostics, protein antibiotics, and biocontrol tools to halt horizontal gene transfer. ### Competing Interest Statement The authors have declared no competing interest. United States Department of Energy, Biopreparedness Research Virtual Environment (BRaVE) Phage Foundry at Lawrence Berkeley National Laboratory is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Biological & Environmental Research under contract number DE-AC02-05CH11231, U.S. Department of Energy Joint Genome Institute, a DOE Office of Science User Facility, is supported by the Office of Science of the U.S. Department of Energy operated under Contract No. DE-AC02-05CH11231, Complex Carbohydrate Research Center was supported in part by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, under award DE-SC0015662, NIH R01GM141659, NIH R24GM137782

Single stranded RNA (ssRNA) and single stranded DNA (ssDNA) bacteriophages represent a key component of the global virome, yet the host genetic networks supporting their infection cycles remain poorly...

Koo et al. show how the yeast Rad51 paralog complex Rad55-Rad57-SHU promotes homologous recombination. Cryo-EM structures reveal that Rad55-Rad57 recruits Rad51 to ssDNA to seed filament formation, while ATP hydrolysis promotes paralog complex turnover.

Somatic mutations in DNMT3A and TET2 are common in age-related clonal haematopoiesis and hematopoietic neoplasms. Here, the authors show that loss of TET2, unlike DNMT3A, makes cells more sensitive to...

Cell Rep · 05 Sep 2026 — Type IV-C CRISPR-Cas systems remain enigmatic compared to other class 1 systems. Here, we expand the type IV-C catalog, identifying two phylogenetically distinct clades primar...

Pittman et al. utilize genetic, biochemical, and structural data to show that type IV-C CRISPR-Cas systems function through DNA targeting. Type IV-C systems do not cleave at the bound target site. Instead, following target binding, the Cas10IVc HD domain nonspecifically cleaves ssDNA and RNA to provide defense against invaders.

Using single-molecule imaging and manipulation, the authors show linker histone H1 preferentially forms phase-separated droplets with single-stranded nucleic acids over double-stranded DNA and nucleos...

In meiosis, homologous recombination (HR) facilitates the halving of genomic content in diploid parents to produce haploid gametes, while also reshuffling genetic material. However, studies in yeast and mammals have suggested that HR during meiosis also makes the process mutagenic, and it has been proposed that the single-strand (ss) DNA formed by HR of programmed double-strand breaks (DSBs) contributes to the observed mutagenesis. To determine the full mutagenic potential of ssDNA formed during meiosis, we expressed human APOBEC3A (A3A), a deaminase that specifically attacks and therefore allows detection of ssDNA, in meiotic yeast cells. We demonstrate that meiotic cells accumulate long tracts of ssDNA manifested by A3A-mutation clusters, of up to 134 mutations spanning more than 25 kb. We show that the formation of mutation clusters during meiosis require Spo11-induced DSBs, and that break-induced replication and hyper-resection of DSBs are the primary mechanisms underlying the form

Read the full paper: PARG inhibition reduces ssDNA levels and limits RPA loading upon replication fork collapse