Nucleic Acids Research

During replication, nucleolytic processing of apurinic/apyrimidinic (AP) sites in single-stranded (ss)DNA is attenuated by the evolutionarily conserved 5-hydroxymethylcytosine binding, embryonic-specific (HMCES) protein. HMCES forms a covalent thiazolidine linkage with the ring-opened aldehyde form of a ssDNA AP site to stabilize the AP site and suppress the formation of DNA double-strand breaks. The resulting HMCES DNA-protein cross-link (DPC) can then be digested by the SPRTN protease and bypassed through mutagenic translesion synthesis (TLS). Here, we use Xenopus egg extracts and molecular dynamics simulations to investigate how HMCES-DPC formation influences the mutagenicity of AP site bypass. We show that SPRTN processes the HMCES-DPC to a five amino acid peptide adduct prior to TLS. Surprisingly, the mutagenicity of HMCES-DPC bypass is insensitive to the extent of DPC proteolysis and depends only on cross-link formation, which traps the AP site in a more dynamic ring-opened configuration. We further show that the spectrum of mutations produced during bypass of HMCES-adducts strongly depends on the template strand nucleotide immediately 5 of the AP site. Our data support a model in which HMCES-DPC formation increases the conformational flexibility of the DNA template, allowing template slippage and use of the 5 template nucleotide to direct insertion opposite the adducted AP site.

The ribosome, long regarded as a passive, uniform machine, has only recently been recognised as a direct regulator of translation. Mass spectrometry and sequencing approaches have shown that heterogeneity in ribosome composition exists, which can actively regulate the translational process. One source of this heterogeneity is the modification of ribosomal RNA (rRNA), primarily pseudouridylation (pseU) and 2'-O-methylation (2OMe), mediated by specific H/ACA and C/D box small nucleolar RNAs (snoRNAs). Here, we investigate how the stoichiometry of rRNA modifications varies during embryonic stem cell differentiation. Using the modification basecalling capability of Nanopore direct RNA sequencing, we have identified distinct stoichiometric changes in modification patterns between pluripotent and differentiated cells, revealing highly dynamic, site-specific regulation. Further, profiling of snoRNA expression during trilineage differentiation revealed differential expression of H/ACA and C/D box snoRNAs responsible for a subset of these dynamic modifications. By integrating rRNA and snoRNA sequencing approaches, we have built a comprehensive profile of rRNA modification dynamics during early embryonic cell fate decisions, highlighting potential regulatory mechanisms for ribosome heterogeneity during development. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=191 HEIGHT=200 SRC="FIGDIR/small/743918v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@48ca91org.highwire.dtl.DTLVardef@eb0418org.highwire.dtl.DTLVardef@159fc8corg.highwire.dtl.DTLVardef@d34f19_HPS_FORMAT_FIGEXP M_FIG C_FIG

Translesion DNA synthesis (TLS) enables DNA replication across damaged DNA and promotes stress-induced mutagenesis that contributes to antibiotic resistance in bacteria. The conserved ImuABC mutasome is essential for TLS in many bacterial species, yet the molecular function of its accessory protein, ImuA, has remained elusive. Here we show that Mycolicibacterium smegmatis ImuA assembles into a hexameric complex, likely arranged as a dimer of trimers, with dual enzymatic activities that reshape current models of its role in DNA damage tolerance. We show that ImuA functions as an ATP-dependent helicase that preferentially unwinds DNA substrates containing single-stranded DNA overhangs and identify amino acids required for both hexamer formation and helicase activity. Unexpectedly, ImuA also possesses ATP-independent 5' exonuclease activity, selectively processing ssDNA substrates with free 5' ends. We show a basic patch on the N-terminus is essential for stabilizing both the nuclease motif and oligomerization. Together, these findings identify ImuA as an active DNA-processing enzyme rather than a passive accessory factor and establish oligomerization as a prerequisite for its function. Our work provides a mechanistic framework for understanding how ImuA may function within the ImuABC mutasome to coordinate DNA processing during translesion synthesis.

Sliding clamp proteins play central roles in DNA metabolism, including replication and repair. The ring-shaped E. coli beta clamp accommodates double-stranded DNA and serves as a platform for proteins involved in multiple DNA transactions. The inner pore of the beta clamp harbors a series of positively charged and polar residues that can bind to the negatively charged backbone of the DNA. These residues are arrayed so that they do not align with the charged phosphates of the DNA backbone. It is hypothesized that this arrangement of these residues provides for the movement of the clamp on DNA as it alternates which residues are bound to the DNA backbone. In this work, we mutated specific charged and polar residues that project into the inner pore of the beta clamp. The beta clamp variants are dimers and have similar thermal stability and in general a similar ability to complement a temperature sensitive strain for growth. One exception was beta-Q149A, which appeared as higher-order species on a native gel although its hydrogen-deuterium exchange pattern measured by mass spectrometry was overall similar to WT beta. These variants all had decreased binding to DNA after loading. Optical tweezers experiments were used to monitor loading on single DNA molecules and measure the rate of beta clamp sliding on DNA. Consistent with the hypothesized role of positively charged residues in the beta inner pore, mutation of one residue resulted in a faster rate of sliding on DNA.

DNA helicases preserve genome stability by unwinding DNA during replication, repair, recombination, and transcription, yet their sites of action in vivo remain difficult to define. Here, we describe a sequencing-based strategy to map helicase activity genome-wide by coupling helicases to the single-stranded DNA-specific activation-induced cytidine deaminase (AID). Deamination of cytosines exposed during helicase-mediated DNA unwinding generates strand-specific mutational footprints that can be detected by whole-genome sequencing at near-nucleotide resolution. Using the Saccharomyces cerevisiae RecQ4-family helicase Hrq1, a functional homolog of human RECQL4, we generated the first genome-wide map of Hrq1 activity. Hrq1-dependent deaminations were highly enriched at RNA polymerase III (RNAPIII)-transcribed genes, particularly tRNA genes, where they occurred predominantly on the transcriptional template strand. This localization was reproducible using both overexpressed Hrq1-AID fusions and an inducible dimerization system that recruited AID to endogenously expressed Hrq1, and it was markedly reduced by helicase-inactivating mutation, indicating that active DNA unwinding underlies the observed signal. Hrq1 associated with nearly all tRNA genes irrespective of transcription level, replication orientation, or proximity to transposable elements, yet deletion or overexpression of Hrq1 did not detectably alter pre-tRNA abundance or RNA polymerase III recycling under the conditions tested. Application of the same approach to the PIF1-family helicase Rrm3 recovered its established enrichment at a subset of highly transcribed, head-on tRNA genes, validating the method. Together, these findings establish AID-mediated mutational footprinting as a general approach for mapping helicase activity in vivo and reveal an unexpected, widespread association of the RecQ4-family helicase Hrq1 with RNAPIII-transcribed genes.

O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=79 SRC="FIGDIR/small/745202v1_ufig1.gif" ALT="Figure 1"> View larger version (21K): org.highwire.dtl.DTLVardef@52e3edorg.highwire.dtl.DTLVardef@1f21637org.highwire.dtl.DTLVardef@909ee8org.highwire.dtl.DTLVardef@b0c907_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGRAPHICAL ABSTRACTC_FLOATNO C_FIG Crosslinking and immunoprecipitation followed by sequencing (CLIP-seq) is widely used to identify the RNA targets of RNA-binding proteins (RBPs). However, its application to non-canonical RBPs lacking canonical RNA-binding domains and frequently displaying low or transient RNA occupancy, is limited by low signal-to-noise ratios, high input requirements and error-prone ligation steps during library preparation. To overcome these limitations, we developed soniCLIP, a streamlined CLIP-seq workflow that replaces RNase-mediated RNA fragmentation with sonication and uses a ligation-free strategy for library construction. soniCLIP is optimized for reproducible identification of enriched RBP-associated RNA regions from limited starting material. We benchmarked soniCLIP against the widespread eCLIP approach and observed reproducible recovery of known RBP-associated regions and target recovery comparable to ENCODE eCLIP, while requiring only 10% (500 {micro}g) of protein input. We further applied soniCLIP to the glycolytic enzyme and non-canonical RBP pyruvate kinase M2 (PKM2). We identified 197 significantly enriched RNA regions and validated selected targets by RIP-qRT-PCR and in vitro binding assays. By combining reduced input requirements, high reproducibility, a shortened 3.5-day workflow and the elimination of gel-based purification, soniCLIP provides an efficient and robust approach for the identification of RNA targets of canonical and non-canonical RBPs.

BACH1 is a heme-regulated basic-leucine-zipper containing transcriptional repressor that binds its DNA recognition elements as a heterodimer with MAFK. Heme-binding is thought to be mediated by several Cys-Proline (CP) motifs and this results in dissociation of the heterodimer from DNA. The mechanism of heme-binding and heme-mediated DNA dissociation remains unresolved. We have used UV-visible spectroscopy, 2D-NMR and DNA-binding assays to explore both heme-binding and DNA dissociation of a minimal BACH1 construct containing 2 CP motifs (C492(CP5) and C646(CP6)) flanking the DNA-binding domain. We find that heme is able to bind to both CP motifs, but also to other non-CP cysteines and histidines in the construct. Using NMR spectroscopy, we identify a structured binding pocket in which heme interacts with both C646(CP6) and Cys621. However, DNA-binding assays show that C646(CP6) is not required for heme-mediated DNA dissociation of the BACH1:MAFK heterodimer. Using UV-visible spectroscopy we show that C492(CP5) also recruits heme with a second ligand, a conserved histidine, His559, in the BACH1 DNA-recognition helix. Mutation of C492(CP5) reduces but does not abolish heme-mediated dissociation from DNA. Our findings suggest a mechanism for heme-binding to BACH1 and heme-mediated dissociation from DNA.

Previous studies revealed a crosstalk between intronic polyadenylation (IPA) and the DNA damage response (DDR). Indeed, genotoxic agents, including radiations and anticancer drugs (e.g., cisplatin that crosslinks DNA), regulate the ratio of IPA to last-exon transcripts in many genes. Conversely, multiple genes involved in the DDR, especially homologous recombination, are regulated at the IPA level. The U1 small nuclear RNA (snRNA) widely represses IPA, thereby enhancing full-length gene transcription. However, besides its implication in IPA regulation by ultraviolet-C radiation, little is known about U1 snRNA effects on the DDR and on cell sensitivity to genotoxic agents. Here, we show that U1 snRNA blockade using an antisense oligonucleotide (U1-AMO) in lung cancer cell lines enhances cell growth inhibition by cisplatin, through an increase in cisplatin-induced DNA damage. 3-seq analysis indicates that U1 snRNA blockade represses full-length mRNA expression of multiple genes of the nucleotide-excision repair and Fanconi anemia pathways, which are involved in the repair of cisplatin-DNA crosslinks. Our 3-seq analyses also reveal that moderate doses of U1-AMO and cisplatin upregulate the IPA:LE isoform ratio in overlapping but distinct sets of genes, and that U1-AMO prevents cisplatin effects on the IPA:LE ratio in a large subset of genes. Altogether, these data extend the crosstalk between IPA and the DDR and suggest that U1 snRNA targeting may be used to sensitize cancer cells to genotoxic agents.

Escherichia coli uses wobble uridine (U34) modifications to tune codon decoding, but how individual tRNA writer enzymes shape gene expression remains unclear. Here, we identify MnmA, the U34 thiolation enzyme for tRNALys, tRNAGln, and tRNAGlu, as a central regulator linking codon-directed translation to regulon control and stress response. Loss of MnmA depleted s2U-dependent wobble modifications, preventing geranyl-(ges2U) and seleno-(se2U)-based modifications, causing growth defects, reduced catalase activity, and multi-level gene expression dysregulation. The {Delta}mnmA cells showed broad adaptive transcriptional reprogramming associated with RpoS- and OxyR-regulated pathways, which was accompanied by compromised protein output. Endogenous and tagged-protein analyses revealed specific impairment of transcriptional regulators, adaptive and detoxification proteins, including RpoS, OxyR, FliA, KatE, and KatG. Polysome profiling and polysome-associated RNA sequencing showed that MnmA deficiency globally reduces translational capacity and uncouples mRNA abundance from translational efficiency, which is exacerbated during oxidative stress. We developed genome-wide codon-usage mapping analytics to identify five codon-defined gene clusters, with specific clusters enriched for Lys, Gln, and Glu codons disproportionately affected by MnmA loss. Together, these findings support that wobble uridine thiolation and downstream modifications pair with corresponding codon architecture to coordinate the translation of regulon controllers and stress-response networks linked to bacterial fitness. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=159 HEIGHT=200 SRC="FIGDIR/small/745044v1_ufig1.gif" ALT="Figure 1"> View larger version (36K): org.highwire.dtl.DTLVardef@1a4058borg.highwire.dtl.DTLVardef@167f4cdorg.highwire.dtl.DTLVardef@1f8c7a8org.highwire.dtl.DTLVardef@1fc0767_HPS_FORMAT_FIGEXP M_FIG C_FIG

Gene knockouts by CRISPR-Cas nucleases rely on targeted DNA cleavage and error-prone DNA repair: end-joining pathways can introduce insertions and deletions that assist in disrupting the coding sequence. However, only a fraction of edits achieves this, and an unfavorable array of repair outcomes typically requires switching to another editing technology. Key factors that influence repair are the types and lengths of DNA ends following cleavage. Here, we investigated Cas12a's ability to produce different ends and if they can be used to redistribute editing outcomes. We determined the sites and rates of target cleavage by Cas12a in vitro by combining kinetic modeling with nucleotide-resolution assays. For the first time, we show that trimming - repeated cleavage of an already cut target - occurs about 4x faster than initial cleavage; it also presents alternative DNA end structures for cellular repair. We next introduced specific mismatches to the gRNA. Cas12a maintained fast target cleavage, but changed where the target was cleaved and how quickly it was trimmed, compared to matched gRNA. We exploited the differences in cleavage dynamics between matched and mismatched gRNAs to develop reprogrammed gRNAs, i.e. rpgRNAs. Intentionally-mismatched rpgRNAs retained the high editing efficiency observed with traditional gRNAs. However, they redirected editing between in-frame and out-of-frame outcomes to enhance gene knockout success across genes. Reprogrammed gRNAs offer an efficient way to steer editing toward such preferred outcomes, while retaining the simplicity of gene editing with CRISPR-Cas nucleases.

UvrD-family Superfamily 1A helicases are processive ATP-dependent motor proteins that function during DNA replication, recombination, repair, and transcription. UvrD-family monomers translocate along single stranded (ss) DNA with 3-to-5 directionality but must be activated by dimerization to become helicases in the absence of force or accessory factors. Mycobacterium tuberculosis (Mtb) UvrD1 helicase forms dimers via a disulfide bond between native cysteines in the 2B sub-domains of each monomer. E. coli UvrD forms non-covalent dimers using the same 2B domain interface as in Mtb UvrD1. Using both ensemble and single DNA molecule approaches we examined an E. coli UvrD variant (R421C), which forms covalent dimers with constitutive helicase activity. For the first time this has enabled us to compare the ssDNA translocation and helicase activities of covalent dimers and monomers. Crosslinked UvrD dimers exhibit much higher ssDNA translocation processivities than monomers, although with similar translocation rates. Crosslinked UvrD dimers also show highly processive DNA unwinding of thousands of base pairs, much higher than non-crosslinked UvrD dimers, while monomers show no DNA unwinding activity. DNA unwinding rates of crosslinked UvrD dimers are only [~]20% slower than ssDNA translocation rates, indicating they are "active" helicases that directly facilitate duplex destabilization.

In eukaryotes, DNA replication requires the origin recognition complex (ORC), a six-subunit assembly that promotes replisome formation on chromosomal origins. Orc6 is the smallest and least evolutionarily conserved among all ORC subunits. In Drosophila, Orc6 binds tightly with the core ORC(1-5) and is required for DNA binding and replication initiation, whereas in Xenopus and human systems Orc6 loosely associates with the rest of the complex resulting in some differences for replication-associated activities. Despite these variations, Orc6 remains essential for viability in all species. In current study we analyzed specific residues within the C-terminal 11 helix that is critical for stable association of Orc6 with the ORC complex in Drosophila. Human Orc6 lacks these residues, however it possesses a strong nuclear localization signal (NLS) that is absent in Drosophilidae. We propose that this NLS drives human protein to the nucleus and compensates for weaker Orc6-ORC(1-5) interactions by increasing the nuclear concentration of Orc6 and shifting the equilibrium toward formation of the fully assembled ORC complex at the DNA.

The sequence-dependent mechanical landscapes of double-stranded nucleic acid (dsNA) remain largely unexplored beyond canonical dsDNA. We describe cgNA+, a coarse-grained predictive model of the mechanics of dsRNA, DNA:RNA hybrids, and epigenetically modified dsDNA, all parameterised from 1.26 milliseconds of atomistic simulations. cgNA+ predicts non-local sequence-dependent equilibrium shape and stiffness with errors an order of magnitude smaller than sequence-variability, while enabling exploration of numbers of sequences inaccessible to atomistic simulation. We show that dsNA equilibrium shape is strongly influenced by flanking sequence up to octamer context, with flexible dimer-steps more context-sensitive. CpG-modification alters equilibrium shape comparable to changes caused by single-nucleotide polymorphisms. Groove width analysis across dsNA decamers reveals strong sequence dependence, reflecting the differing characteristic helical geometry of dsDNA and dsRNA, whereas DRHs exhibit mixed behaviour depending on DNA-strand pyrimidine content. CTCF binding sites exhibit a distinct groove width signature. Persistence-length spectra from [~] 9 million sequences indicate that dsRNA is stiffer than dsDNA, whereas DRH exhibit intermediate stiffness modulated by DNA strand pyrimidine content. Persistence length increases upon CpG-modification, but decreases on hypermodification. Overall, the cgNA+ model enables a first, highly accurate, very large-scale, comparative study of sequence-dependent mechanics both within and across dsNA classes, demonstrating previously hidden regulatory layers. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=74 SRC="FIGDIR/small/740023v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@6de6acorg.highwire.dtl.DTLVardef@1435181org.highwire.dtl.DTLVardef@9c2b60org.highwire.dtl.DTLVardef@e3abf0_HPS_FORMAT_FIGEXP M_FIG C_FIG

The problem of how protein sequences translate into defined functions remains largely unsolved despite decades of progress. New methods to efficiently explore protein sequence space will help to shed light on these sequence-function relationships, particularly for complex protein function. Here, we describe an approach to create novel, functional proteins through the integration of deep mutational scanning, structural analysis, and evolutionary mining within prompts for a generative protein language model (PLM). We demonstrate the utility of this approach with the generation of novel compact RNA-guided nucleases. This approach is highly efficient, resulting in active nucleases with [~]40% sequence divergence relative to natural proteins and activity equivalent to or exceeding by up to [~]3X that of other compact nucleases at multiple endogenous loci in human cells. The approach described here is rapidly deployable and produces new sequences that will serve as scaffolds for further exploration of complex protein functionality, as well as substrates for novel genome engineering applications.

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression primarily in the cytoplasm. However, emerging evidence highlights their additional roles in the nucleus. In particular, spliceosomal miRNAs have been implicated in novel regulatory functions, including the modulation of gene expression. Here, we investigate the nuclear role of spliceosomal miR-99b in breast cancer cells, focusing on its interaction with the long non-coding RNA (lncRNA) SPACA6-AS1. Using non-tumorigenic (MCF-10A) and breast cancer cell lines (MCF-7 and MDA-MB-231), we demonstrate that spliceosomal miR-99b expression increases with malignancy and correlates with elevated SPACA6-AS1 pre-mRNA levels. Notably, miR-99b exhibits full complementarity to the 5-prime splice junction of SPACA6-AS1, suggesting a direct role in splicing regulation. Functional assays reveal that inhibition of miR-99b reduces SPACA6-AS1 pre-mRNA levels, whereas its overexpression enhances pre-mRNA accumulation, indicating that miR-99b promotes the formation or stabilization of the unspliced transcript. Furthermore, increased miR-99b expression is associated with altered ratios of SPACA6 isoforms, supporting a broader role in RNA-level regulation of gene expression. Phenotypically, miR-99b enhances breast cancer cell migration and is required for efficient invasion, particularly in highly aggressive cancerous cells. Our findings uncover a novel nuclear function of miR-99b in modulating lncRNA splicing and gene expression. This spliceosomal miR-99b-SPACA6-AS1 axis represents a previously unrecognized regulatory pathway that contributes to breast cancer progression and may provide a potential target for diagnostic and therapeutic strategies.

Oxidative DNA damage is a common threat to genomic integrity, arising from endogenous metabolic processes and environmental exposures. If unrepaired, such oxidative DNA damage promotes mutagenesis and genomic instability. Cells counter this through base excision repair (BER), a multi-step pathway requiring the coordinated action of several proteins. Central to BER, DNA polymerase beta (pol ) locates single-nucleotide (1-nt) gaps and inserts the correct nucleotide, while x-ray repair cross-complementing 1 (XRCC1) is a scaffold protein that forms a stable complex with pol to coordinate BER factors at DNA damage. XRCC1 enhances BER efficiency, though the mechanism by which this occurs is unclear. Pol {beta} is proposed to be recruited to DNA damage by undamaged DNA scanning interactions, but this behavior has not yet been directly observed. Additionally, the influence of other BER proteins on pol recruitment, particularly XRCC1, remains unclear. Here, we used correlative optical tweezers-fluorescence microscopy to visualize DNA search and damage recognition by pol and XRCC1. We characterize each factor individually, examine their behavior as the pol -XRCC1 complex, and assess their interplay with apurinic/apyrimidinic endonuclease 1 (APE1), the enzyme upstream of pol in BER. We find that pol locates damage through 3D-diffusion, whereas XRCC1 exhibits both 3D- and 1D-diffusion. In combination, XRCC1 dramatically shifts pol {beta} search towards 1D-diffusion, enabling interrogation of non-damaged DNA using both search mechanisms. When both APE1 and pol are present, the pol -1nt gap complex is highly stable, with APE1 largely unable to disrupt the damage-bound pol . Together, these findings demonstrate that XRCC1 reshapes pol {beta} search behavior to promote efficient local damage recognition, providing a mechanistic basis for how BER factors coordinate lesion detection and processing to maintain genomic stability.

Precise characterization of genome editing outcomes remains a major challenge because edited cell populations contain diverse alleles generated by homology-directed repair, non-homologous end joining (NHEJ), or base editing. While next-generation sequencing enables comprehensive analysis, its routine use is constrained by cost and turnaround time. Here, we developed a multi-color droplet digital PCR (ddPCR) assay that exploits six-color fluorescence detection to quantitatively distinguish multiple edited alleles within a single reaction. Using CRISPR-Cas9 and base editing model systems, we designed sequence-specific probe sets that distinguished recurrent NHEJ alleles generated by CRISPR-Cas9 editing as well as target and bystander alleles generated by base editing. The assay quantitatively resolved individual editing outcomes that could not be distinguished by conventional Sanger sequencing. Together, these results establish multi-color ddPCR as a rapid, scalable, and sequence-specific approach for quantification of genome editing outcomes across multiple editing modalities.

Tandem interspaced guide RNA (TIGR)-TIGR-associated (Tas) are RNA-guided defense systems, which use a dual-repeat or stem-loop tigRNA to direct a Tas dimer to DNA targets through RNA-DNA heteroduplex formation between both DNA strands. While previous work has shown the basic principles of DNA targeting, how the guide architecture influences target recognition and whether recognition and cleavage are coordinated across the two RNA-DNA heteroduplexes at a target site remain poorly understood. Here, we combine cryo-electron microscopy, biochemistry, and cell-based assays to investigate two TIGR-Tas effectors: the nuclease-lacking Peromyscus leucopus TasA (PlTasA), associated with a stem-loop tigRNA, and the nuclease-active Salicola phage TasH (SpTasH). Cryo-EM structures of PlTasA binary and ternary complexes reveal a dimeric scaffold similar to SpTasH and TaTasR with a distinct stem-loop tigRNA architecture and additional peripheral structural elements. Binding assays using DNA substrates containing local bubbles across the spacer-matching region show equivalent bubbles produced position- and spacer-dependent effects, indicating that target engagement is asymmetric in both PlTasA and SpTasH. Kinetic and cryo-EM analyses of SpTasH further reveal stepwise heteroduplex formation, with a partially engaged intermediate that undergoes substantial conformational rearrangements of the second protomer, preceding a fully paired state poised for catalytic activation. Cleavage of the two DNA strands occurs through a coordinated, slow process and productive cleavage requires stringent surveillance of both heteroduplexes. Together, these findings define a conserved, ordered mechanism of bipartite target recognition and activation shared by TIGR-Tas effectors, expanding our understanding of the molecular principles underlying programmable DNA targeting by TIGR-Tas systems.

Strong evolutionary selection has maintained CpG-dense islands (CGIs) at the promoters of constitutively expressed genes throughout the vertebrate genome, suggesting an important role in regulating DNA topology. Here, using Twist-seq, a psoralen-based approach for quantitative genome-wide profiling of DNA supercoiling, we reveal distinct topological states across human gene promoters. We show that CGI promoters accumulate elevated levels of negative supercoiling relative to non-CGI promoters and define localised topological domains at highly transcribed genes. Integrating genome-wide analyses with reaction-diffusion modelling and coarse-grained molecular dynamics simulations, we find that this behaviour is encoded by the intrinsic physical properties of CGI DNA. The GC-rich sequence context promotes nucleosome depletion and focuses torsional stress onto embedded AT-rich pockets, driving localised DNA melting and plectoneme-tip bubble formation within promoter-proximal nucleosome-free regions. This provides an energetically favourable pathway for redistributing transcription-induced torsional stress through transient strand separation and writhe, consistent with increased ssDNA formation at CGI promoters observed by ssDNA-seq. We propose that CGIs function as sequence-encoded topological sinks that buffer supercoiling while maintaining a promoter architecture permissive for transcription initiation, thereby preserving promoter integrity and genome stability.

The QxxR motif is evolutionarily conserved within DEAD-box RNA helicases, including Drosophila Me31B and human DDX6, which post-transcriptionally regulate gene expression during animal development. A pathogenic H372R substitution (QxHR to QxRR) in the QxxR motif of human DDX6 has been associated with various developmental defects, but how this motif contributes to DDX6-family protein function remains unclear. Here, we used Drosophila Me31B as an in vivo model to investigate the QxxR motifs developmental role. We generated a Drosophila strain carrying the corresponding H333R missense mutation in Me31B and characterized its effects on female fertility, embryonic viability, germline development, and Me31B-associated molecular pathways. The me31BH333R mutation reduced female fertility in a gene dose-dependent manner, with homozygous mutant females being sterile. Embryos from the mutant females also exhibited primordial germ cell defects. Despite these developmental phenotypes, the me31BH333R mutation did not significantly alter Me31B protein abundance, global ovarian transcriptome or proteome profiles, or representative germ plasm mRNA and protein localization. In contrast, bait-normalized IP-MS analysis revealed altered enrichment of selected Me31B-associated proteins, including increased association of known Me31B interactors Trailer hitch (Tral) and Ypsilon Schachtel (Yps). These findings establish Me31BH333R as an in vivo model for investigating the conserved QxxR motif and suggest that disruption of this motif compromises development not through broad changes in gene expression, but potentially through altered composition or regulation of Me31B-containing ribonucleoprotein complexes.