Genomic mapping of phosphorothioates reveals partial modification of short consensus sequences

Phosphorothioate DNA modification by BREX Type 4 systems in the human gut microbiome

Among dozens of microbial DNA modifications regulating gene expression and host defense, phosphorothioation (PT) is the only known backbone modification, with sulfur inserted at a non-bridging oxygen by dnd and ssp gene families. Here we explored the distribution of PT genes in 13,663 human gut microbiome genomes, finding that 6.3% possessed dnd or ssp genes predominantly in Bacillota, Bacteroidota, and Pseudomonadota. This analysis uncovered several putative new PT synthesis systems, including Type 4 Bacteriophage Exclusion (BREX) brx genes, which were genetically validated in Bacteroides salyersiae. Mass spectrometric analysis of DNA from 226 gut microbiome isolates possessing dnd, ssp, and brx genes revealed 8 PT dinucleotide settings confirmed in 6 consensus sequences by PT-specific DNA sequencing. Genomic analysis showed PT enrichment in rRNA genes and depletion at gene boundaries. These results illustrate the power of the microbiome for discovering prokaryotic epigenetics and the widespread distribution of oxidation-sensitive PTs in gut microbes.

PT-seq: A method for metagenomic analysis of phosphorothioate epigenetics in complex microbial communities

Among dozens of known epigenetic marks, naturally occurring phosphorothioate (PT) DNA modifications are unique in replacing a non-bridging phosphate oxygen with redox-active sulfur and function in prokaryotic restriction-modification and transcriptional regulation. Interest in PTs has grown due to the widespread distribution of the dnd, ssp, and brx genes among bacteria and archaea, as well as the discovery of PTs in 5-10% of gut microbes. Efforts to map PTs in complex microbiomes using existing next-generation and direct sequencing technologies have failed due to poor sensitivity. Here we developed PT-seq as a high-sensitivity method to quantitatively map PTs across genomes and metagenomically identify PT-containing microbes in complex genomic mixtures. Like other methods for mapping PTs in individual genomes, PT-seq exploits targeted DNA strand cleavage at PTs by iodine, followed by sequencing library construction using ligation or template switching approaches. However, PT-specific sequencing reads are dramatically increased by adding steps to heat denature the DNA, block pre-existing 3'-ends, fragment DNA after T-tailing, and enrich iodine-induced breaks using biotin-labeling and streptavidin beads capture. Iterative optimization of the sensitivity and specificity of PT-seq is demonstrated with individual bacteria and human fecal DNA.

Temporal dynamics and metagenomics of phosphorothioate epigenomes in the human gut microbiome

Background

Epigenetic regulation of gene expression and host defense is well established in microbial communities, with dozens of DNA modifications comprising the epigenomes of prokaryotes and bacteriophage. Phosphorothioation (PT) of DNA, in which a chemically-reactive sulfur atom replaces a non-bridging oxygen in the sugar-phosphate backbone, is catalyzed by dnd and ssp gene families widespread in bacteria and archaea. However, little is known about the role of PTs or other microbial epigenetic modifications in the human microbiome. Here we optimized and applied fecal DNA extraction, mass spectrometric, and metagenomics technologies to characterize the landscape and temporal dynamics of gut microbes possessing PT modifications.

Results

Exploiting the nuclease-resistance of PTs, mass spectrometric analysis of limit digests of PT-containing DNA reveals PT dinucleotides as part of genomic consensus sequences, with 16 possible dinucleotide combinations. Analysis of mouse fecal DNA revealed a highly uniform spectrum of 11 PT dinucleotides in all littermates, with PTs estimated to occur in 5-10% of gut microbes. Though at similar levels, PT dinucleotides in fecal DNA from 11 healthy humans possessed signature combinations and levels of individual PTs. Comparison with a widely distributed microbial epigenetic mark, m6dA, suggested temporal dynamics consistent with expectations for gut microbial communities based on Taylor's Power Law. Application of PT-seq for site-specific metagenomic analysis of PT-containing bacteria in one fecal donor revealed the larger consensus sequences for the PT dinucleotides in Bacteroidota, Firmicutes, Actinobacteria, and Proteobacteria, which differed from unbiased metagenomics and suggested that the abundance of PT-containing bacteria did not simply mirror the spectrum of gut bacteria. PT-seq further revealed low abundance PT sites not detected as dinucleotides by mass spectrometry, attesting to the complementarity of the technologies.

Conclusions

The results of our studies provide a benchmark for understanding the behavior of an abundant and chemically-reactive epigenetic mark in the human gut microbiome, with implications for inflammatory conditions of the gut.

The binding affinity-dependent inhibition of cell growth and viability by DNA sulfur-binding domains

Increasing evidence suggests that DNA phosphorothioate (PT) modification serves several purposes in the bacterial host, and some restriction enzymes specifically target PT-DNA. PT-dependent restriction enzymes (PDREs) bind PT-DNA through their DNA sulfur binding domain (SBD) with dissociation constants (KD) of 5 nM~1 μM. Here, we report that SprMcrA, a PDRE, failed to dissociate from PT-DNA after cleavage due to high binding affinity, resulting in low DNA cleavage efficiency. Expression of SBDs in Escherichia coli cells with PT modification induced a drastic loss of cell viability at 25°C when both DNA strands of a PT site were bound, with one SBD on each DNA strand. However, at this temperature, SBD binding to only one PT DNA strand elicited a severe growth lag rather than lethality. This cell growth inhibition phenotype was alleviated by raising the growth temperature. An in vitro assay mimicking DNA replication and RNA transcription demonstrated that the bound SBD hindered the synthesis of new DNA and RNA when using PT-DNA as the template. Our findings suggest that DNA modification-targeting proteins might regulate cellular processes involved in DNA metabolism in addition to being components of restriction-modification systems and epigenetic readers.

Reappraisal of the DNA phosphorothioate modification machinery: uncovering neglected functional modalities and identification of new counter-invader defense systems

The DndABCDE systems catalysing the unusual phosphorothioate (PT) DNA backbone modification, and the DndFGH systems, which restrict invasive DNA, have enigmatic and paradoxical features. Using comparative genomics and sequence-structure analyses, we show that the DndABCDE module is commonly functionally decoupled from the DndFGH module. However, the modification gene-neighborhoods encode other nucleases, potentially acting as the actual restriction components or suicide effectors limiting propagation of the selfish elements. The modification module's core consists of a coevolving gene-pair encoding the DNA-scanning apparatus - a DndD/CxC-clade ABC ATPase and DndE with two ribbon-helix-helix (MetJ/Arc) DNA-binding domains. Diversification of DndE's DNA-binding interface suggests a multiplicity of target specificities. Additionally, many systems feature DNA cytosine methylase genes instead of PT modification, indicating the DndDE core can recruit other nucleobase modifications. We show that DndFGH is a distinct counter-invader system with several previously uncharacterized domains, including a nucleotide kinase. These likely trigger its restriction endonuclease domain in response to multiple stimuli, like nucleotides, while blocking protective modifications by invader methylases. Remarkably, different DndH variants contain a HerA/FtsK ATPase domain acquired from multiple sources, including cellular genome-segregation systems and mobile elements. Thus, we uncovered novel HerA/FtsK-dependent defense systems that might intercept invasive DNA during replication, conjugation, or packaging.

Understanding base and backbone contributions of phosphorothioate DNA for molecular recognition with SBD proteins

Bacterial DNA phosphorothioate (PT) modification provides a specific anchoring site for sulfur-binding proteins (SBDs). Besides, their recognition patterns include phosphate links and bases neighboring the PT-modified site, thereby bringing about genome sequence-dependent properties in PT-related epigenetics. Here, we analyze the contributions of the DNA backbone (phosphates and deoxyribose) and bases bound with two SBD proteins in Streptomyces pristinaespiralis and coelicolor (SBDSco and SBDSpr). The chalcogen-hydrophobic interactions remained constantly at the anchoring site while the adjacent bases formed conditional and distinctive non-covalent interactions. More importantly, SBD/PT-DNA interactions were not limited within the traditional "4-bp core" range from 5'-I to 3'-III but extended to upstream 5'-II and 5'-III bases and even 5''-I to 5''-III at the non-PT-modified complementary strand. From the epigenetic viewpoint, bases 3'-II, 5''-I, and 5''-III of SBDSpr and 3'-II, 5''-II, and 5''-III of SBDSco present remarkable differentiations in the molecular recognitions. From the protein viewpoint, H102 in SBDSpr and R191 in SBDSco contribute significantly while proline residues at the PT-bound site are strictly conserved for the PT-chalcogen bond. The mutual and make-up mutations are proposed to alter the SBD/PT-DNA recognition pattern, besides additional chiral phosphorothioate modifications on phosphates 5'-II, 5'-II, 3'-I, and 3'-II.

A DNA phosphorothioation-based Dnd defense system provides resistance against various phages and is compatible with the Ssp defense system

DndABCDE-catalyzed DNA phosphorothioation (PT), in which the nonbridging oxygen is swapped with a sulfur atom, was first identified in the bacterial genome. Usually, this modification gene cluster is paired with a restriction module consisting of DndF, DndG, and DndH. Although the mechanisms for the antiphage activity conferred by this Dnd-related restriction and modification (R-M) system have been well characterized, several features remain unclear, including the antiphage spectrum and potential interference with DNA methylation. Recently, a novel PT-related R-M system, composed of the modification module SspABCD paired with a single restriction enzyme, SspE, was revealed to be widespread in the bacterial kingdom, which aroused our interest in the interaction between Dnd- and Ssp-based R-M systems. In this study, we discussed the action of Dnd-related R-M systems against phages and demonstrated that the host could benefit from the protection provided by Dnd-related R-M systems against infection by various lytic phages as well as temperate phages. However, this defense barrier would fail against lysogenic phages. Interestingly, DNA methylation, even in the consensus sequence recognized by the Dnd system, could not weaken the restriction efficiency. Finally, we explored the interaction between Dnd- and Ssp-based R-M systems and found that these two systems were compatible. This study not only expands our knowledge of Dnd-associated R-M systems but also reveals a complex interaction between different defense barriers that coexist in the cell. IMPORTANCE Recently, we decoded the mechanism of Dnd-related R-M systems against genetic parasites. In the presence of exogenous DNA that lacks PT, the macromolecular machine consisting of DndF, DndG, and DndH undergoes conformational changes to perform DNA binding, translocation, and DNA nicking activities and scavenge the foreign DNA. However, several questions remain unanswered, including questions regarding the antiphage spectrum, potential interference by DNA methylation, and interplay with other PT-dependent R-M systems. Here, we revealed that the host could benefit from Dnd-related R-M systems for a broad range of antiphage activities, regardless of the presence of DNA methylation. Furthermore, we demonstrated that the convergence of Dnd- and Ssp-related R-M systems could confer to the host a stronger antiphage ability through the additive suppression of phage replication. This study not only deepens our understanding of PT-related defense barriers but also expands our knowledge of the arms race between bacteria and their predators.

Recurring and emerging themes in prokaryotic innate immunity

A resurgence of interest in the pathways that bacteria use to protect against their viruses (i.e. phages) has led to the discovery of dozens of new antiphage defenses. Given the sheer abundance and diversity of phages - the ever-evolving targets of immunity - it is not surprising that these newly described defenses are also remarkably diverse. However, as their mechanisms slowly come into focus, some common strategies and themes are also beginning to emerge. This review highlights recurring and emerging themes in the mechanisms of innate immunity in bacteria and archaea, with an emphasis on recently described systems that have undergone more thorough mechanistic characterization.

The DNA Phosphorothioation Restriction-Modification System Influences the Antimicrobial Resistance of Pathogenic Bacteria

Bacterial defense barriers, such as DNA methylation-associated restriction-modification (R-M) and the CRISPR-Cas system, play an important role in bacterial antimicrobial resistance (AMR). Recently, a novel R-M system based on DNA phosphorothioate (PT) modification has been shown to be widespread in the kingdom of Bacteria as well as Archaea. However, the potential role of the PT R-M system in bacterial AMR remains unclear. In this study, we explored the role of PT R-Ms in AMR with a series of common clinical pathogenic bacteria. By analyzing the distribution of AMR genes related to mobile genetic elements (MGEs), it was shown that the presence of PT R-M effectively reduced the distribution of horizontal gene transfer (HGT)-derived AMR genes in the genome, even in the bacteria that did not tend to acquire AMR genes by HGT. In addition, unique gene variation analysis based on pangenome analysis and MGE prediction revealed that the presence of PT R-M could suppress HGT frequency. Thus, this is the first report showing that the PT R-M system has the potential to repress HGT-derived AMR gene acquisition by reducing the HGT frequency. IMPORTANCE In this study, we demonstrated the effect of DNA PT modification-based R-M systems on horizontal gene transfer of AMR genes in pathogenic bacteria. We show that there is no apparent association between the genetic background of the strains harboring PT R-Ms and the number of AMR genes or the kinds of gene families. The strains equipped with PT R-M harbor fewer plasmid-derived, prophage-derived, or integrating mobile genetic element (iMGE)-related AMR genes and have a lower HGT frequency, but the degree of inhibition varies among different bacteria. In addition, compared with Salmonella enterica and Escherichia coli, Klebsiella pneumoniae prefers to acquire MGE-derived AMR genes, and there is no coevolution between PT R-M clusters and bacterial core genes.

Nicking mechanism underlying the DNA phosphorothioate-sensing antiphage defense by SspE

DNA phosphorothioate (PT) modification, with a nonbridging phosphate oxygen substituted by sulfur, represents a widespread epigenetic marker in prokaryotes and provides protection against genetic parasites. In the PT-based defense system Ssp, SspABCD confers a single-stranded PT modification of host DNA in the 5'-C_PSCA-3' motif and SspE impedes phage propagation. SspE relies on PT modification in host DNA to exert antiphage activity. Here, structural and biochemical analyses reveal that SspE is preferentially recruited to PT sites mediated by the joint action of its N-terminal domain (NTD) hydrophobic cavity and C-terminal domain (CTD) DNA binding region. PT recognition enlarges the GTP-binding pocket, thereby increasing GTP hydrolysis activity, which subsequently triggers a conformational switch of SspE from a closed to an open state. The closed-to-open transition promotes the dissociation of SspE from self PT-DNA and turns on the DNA nicking nuclease activity of CTD, enabling SspE to accomplish self-nonself discrimination and limit phage predation, even when only a small fraction of modifiable consensus sequences is PT-protected in a bacterial genome.

The red thread between methylation and mutation in bacterial antibiotic resistance: How third-generation sequencing can help to unravel this relationship

DNA methylation is an important mechanism involved in bacteria limiting foreign DNA acquisition, maintenance of mobile genetic elements, DNA mismatch repair, and gene expression. Changes in DNA methylation pattern are observed in bacteria under stress conditions, including exposure to antimicrobial compounds. These changes can result in transient and fast-appearing adaptive antibiotic resistance (AdR) phenotypes, e.g., strain overexpressing efflux pumps. DNA methylation can be related to DNA mutation rate, because it is involved in DNA mismatch repair systems and because methylated bases are well-known mutational hotspots. The AdR process can be the first important step in the selection of antibiotic-resistant strains, allowing the survival of the bacterial population until more efficient resistant mutants emerge. Epigenetic modifications can be investigated by third-generation sequencing platforms that allow us to simultaneously detect all the methylated bases along with the DNA sequencing. In this scenario, this sequencing technology enables the study of epigenetic modifications in link with antibiotic resistance and will help to investigate the relationship between methylation and mutation in the development of stable mechanisms of resistance.

Full-length transcriptome-referenced analysis reveals crucial roles of hormone and wounding during induction of aerial bulbils in lily

Aerial bulbils are important vegetative reproductive organs in Lilium. They are often perpetually dormant in most Lilium species, and little is known about the induction of these vegetative structures. The world-famous Oriental hybrid lily cultivar 'Sorbonne', which blooms naturally devoid of aerial bulbils, is known for its lovely appearance and sweet fragrance. We found that decapitation stimulated the outgrowth of aerial bulbils at lower stems (LSs) and then application of low and high concentrations of IAA promoted aerial bulbils emergence around the wound at upper stems (USs) of 'Sorbonne'. However, the genetic basis of aerial bulbil induction is still unclear. Herein, 'Sorbonne' transcriptome has been sequenced for the first time using the combination of third-generation long-read and next-generation short-read technology. A total of 46,557 high-quality non-redundant full-length transcripts were generated. Transcriptomic profiling was performed on seven tissues and stems with treatments of decapitation and application of low and high concentrations of IAA, respectively. Functional annotation of 1918 DEGs within stem samples of different treatments showed that hormone signaling, sugar metabolism and wound-induced genes were crucial to bulbils outgrowth. The expression pattern of auxin-, shoot branching hormone-, plant defense hormone- and wound-inducing-related genes indicated their crucial roles in bulbil induction. Then we established five hormone- and wounding-regulated co-expression modules and identified some candidate transcriptional factors, such as MYB, bZIP, and bHLH, that may function in inducing bulbils. High connectivity was observed among hormone signaling genes, wound-induced genes, and some transcriptional factors, suggesting wound- and hormone-invoked signals exhibit extensive cross-talk and regulate bulbil initiation-associated genes via multilayered regulatory cascades. We propose that the induction of aerial bulbils at LSs after decapitation can be explained as the release of apical dominance. In contrast, the induction of aerial bulbils at the cut surface of USs after IAA application occurs via a process similar to callus formation. This study provides abundant candidate genes that will deepen our understanding of the regulation of bulbil outgrowth, paving the way for further molecular breeding of lily.

Microfluidics-Assisted Fluorescence Mapping of DNA Phosphorothioation

As the key player of a new restriction modification system, DNA phosphorothioate (PT) modification, which swaps oxygen for sulfur on the DNA backbone, protects the bacterial host from foreign DNA invasion. The identification of PT sites helps us understand its physiological defense mechanisms, but accurately quantifying this dynamic modification remains a challenge. Herein, we report a simple quantitative analysis method for optical mapping of PT sites in the single bacterial genome. DNA molecules are fully stretched and immobilized in a microfluidic chip by capillary flow and electrostatic interactions, improving the labeling efficiency by maximizing exposure of PT sites on DNA while avoiding DNA loss and damage. After screening 116 candidates, we identified a bifunctional chemical compound, iodoacetyl-polyethylene glycol-biotin, that can noninvasively and selectively biotinylate PT sites, enabling further labeling with streptavidin fluorescent nanoprobes. With this method, PT sites in PT⁺ DNA can be easily detected by fluorescence, while almost no detectable ones were found in PT^- DNA, achieving real-time visualization of PT sites on a single DNA molecule. Collectively, this facile genome-wide PT site detection method directly characterizes the distribution and frequency of DNA modification, facilitating a better understanding of its modification mechanism that can be potentially extended to label DNAs in different species.

Origin of iodine preferential attack at sulfur in phosphorothioate and subsequent P-O or P-S bond dissociation

Iodine-induced cleavage at phosphorothioate DNA (PT-DNA) is characterized by extremely high sensitivity (∼1 phosphorothioate link per 106 nucleotides), which has been used for detecting and sequencing PT-DNA in bacteria. Despite its foreseeable potential for wide applications, the cleavage mechanism at the PT-modified site has not been well established, and it remains unknown as to whether or not cleavage of the bridging P-O occurs at every PT-modified site. In this work, we conducted accurate ωB97X-D calculations and high-performance liquid chromatography-mass spectrometry to investigate the process of PT-DNA cleavage at the atomic and molecular levels. We have found that iodine chemoselectively binds to the sulfur atom of the phosphorothioate link via a strong halogen-chalcogen interaction (a type of halogen bond, with binding affinity as high as 14.9 kcal/mol) and thus triggers P-O bond cleavage via phosphotriester-like hydrolysis. Additionally, aside from cleavage of the bridging P-O bond, the downstream hydrolyses lead to unwanted P-S/P-O conversions and a loss of the phosphorothioate handle. The mechanism we outline helps to explain specific selectivity at the PT-modified site but also predicts the dynamic stoichiometry of P-S and P-O bond breaking. For instance, Tris is involved in the cascade derivation of S-iodo-phosphorothioate to S-amino-phosphorothioate, suppressing the S-iodo-phosphorothioate hydrolysis to a phosphate diester. However, hydrolysis of one-third of the Tris-O-grafting phosphotriester results in unwanted P-S/P-O conversions. Our study suggests that bacterial DNA phosphorothioation may more frequently occur than previous bioinformatic estimations have predicted from iodine-induced deep sequencing data.

Nanopore Sequencing for Detection and Characterization of Phosphorothioate Modifications in Native DNA Sequences

Bacterial DNA is subject to various modifications involved in gene regulation and defense against bacteriophage attacks. Phosphorothioate (PT) modifications are protective modifications in which the non-bridging oxygen in the DNA phosphate backbone is replaced with a sulfur atom. Here, we expand third-generation sequencing techniques to allow for the sequence-specific mapping of DNA modifications by demonstrating the application of Oxford Nanopore Technologies (ONT) and the ELIGOS software package for site-specific detection and characterization of PT modifications. The ONT/ELIGOS platform accurately detected PT modifications in a plasmid carrying synthetic PT modifications. Subsequently, studies were extended to the genome-wide mapping of PT modifications in the Salmonella enterica genomes within the wild-type strain and strains lacking the PT regulatory gene dndB (ΔdndB) or the PT synthetic gene dndC (ΔdndC). PT site-specific signatures were observed in the established motifs of GAAC/GTTC. The PT site locations were in close agreement with PT sites previously identified using the Nick-seq technique. Compared to the wild-type strain, the number of PT modifications are 1.8-fold higher in ΔdndB and 25-fold lower in ΔdndC, again consistent with known regulation of the dnd operon. These results demonstrate the suitability of the ONT platform for accurate detection and identification of the unusual PT backbone modifications in native genome sequences.

Involvement of the DNA Phosphorothioation System in TorR Binding and Anaerobic TMAO Respiration in Salmonella enterica

Although the phosphorothioate (PT) modification, in which the nonbridging oxygen in the DNA sugar-phosphate backbone is replaced by sulfur, has been reported to play versatile roles in multiple cellular processes, very little data have been obtained to define the role of PT in epigenetic regulation. In this study, we report that the PT system in Salmonella enterica serovar Cerro 87 is involved in the transcriptional regulation of the torCAD operon encoding the trimethylamine N-oxide (TMAO) respiration machinery that enables the use of TMAO as a terminal electron acceptor for respiration when oxygen is not available. In vitro, PT enhanced the binding of the transcriptional activator of the torCAD operon, namely, TorR, to its DNA substrate (tor boxes). However, in vivo, the PT modification protein complex DndCDE downregulated torCAD transcription through competing with the binding of TorR to the tor boxes. The altered expression of torCAD caused by PT modification proteins affected cell growth that relied on TMAO respiration. To our knowledge, this is the first report supporting that PT proteins participate in transcriptional regulation, showing a new function of PT systems.

Structural and Functional Analysis of DndE Involved in DNA Phosphorothioation in the Haloalkaliphilic Archaea Natronorubrum bangense JCM10635

Phosphorothioate (PT) modification, a sequence-specific modification that replaces the nonbridging oxygen atom with sulfur in a DNA phosphodiester through the gene products of dndABCDE or sspABCD, is widely distributed in prokaryotes. DNA PT modification functions together with gene products encoded by dndFGH, pbeABCD, or sspE to form defense systems that can protect against invasion by exogenous DNA particles. While the functions of the multiple enzymes in the PT system have been elucidated, the exact role of DndE in the PT process is still obscure. Here, we solved the crystal structure of DndE from the haloalkaliphilic archaeal strain Natronorubrum bangense JCM10635 at a resolution of 2.31 Å. Unlike the tetrameric conformation of DndE in Escherichia coli B7A, DndE from N. bangense JCM10635 exists in a monomeric conformation and can catalyze the conversion of supercoiled DNA to nicked or linearized products. Moreover, DndE exhibits preferential binding affinity to nicked DNA by virtue of the R19- and K23-containing positively charged surface. This work provides insight into how DndE functions in PT modification and the potential sulfur incorporation mechanism of DNA PT modification.

Revisiting the Extinction of the RNA World

The ribozyme world is thought to have evolved the burdensome complexity of peptide and protein synthesis because the 20 amino acid side chains are catalytically superior. Instead, I propose that the Achilles heel of the RNA world that led to the extinction of riboorganisms was RNA’s polyanionic charges that could not be covalently neutralized stably by phosphotriester formation. These charges prevented development of hydrophobic cores essential for integration into membranes and many enzymatic reactions. In contrast, the phosphotriester modification of DNA is stable. So, the fact that the charge was never removed in DNA evolution gives further credence to proteins coming before DNA.

Diverse DNA modification in marine prokaryotic and viral communities

DNA chemical modifications, including methylation, are widespread and play important roles in prokaryotes and viruses. However, current knowledge of these modification systems is severely biased towards a limited number of culturable prokaryotes, despite the fact that a vast majority of microorganisms have not yet been cultured. Here, using single-molecule real-time sequencing, we conducted culture-independent 'metaepigenomic' analyses (an integrated analysis of metagenomics and epigenomics) of marine microbial communities. A total of 233 and 163 metagenomic-assembled genomes (MAGs) were constructed from diverse prokaryotes and viruses, respectively, and 220 modified motifs and 276 DNA methyltransferases (MTases) were identified. Most of the MTase genes were not genetically linked with the endonuclease genes predicted to be involved in defense mechanisms against extracellular DNA. The MTase-motif correspondence found in the MAGs revealed 10 novel pairs, 5 of which showed novel specificities and experimentally confirmed the catalytic specificities of the MTases. We revealed novel alternative specificities in MTases that are highly conserved in Alphaproteobacteria, which may enhance our understanding of the co-evolutionary history of the methylation systems and the genomes. Our findings highlight diverse unexplored DNA modifications that potentially affect the ecology and evolution of prokaryotes and viruses in nature.

Current and Future Methodology for Quantitation and Site-Specific Mapping the Location of DNA Adducts

Formation of DNA adducts is a key event for a genotoxic mode of action, and their presence is often used as a surrogate for mutation and increased cancer risk. Interest in DNA adducts are twofold: first, to demonstrate exposure, and second, to link DNA adduct location to subsequent mutations or altered gene regulation. Methods have been established to quantitate DNA adducts with high chemical specificity and to visualize the location of DNA adducts, and elegant bio-analytical methods have been devised utilizing enzymes, various chemistries, and molecular biology methods. Traditionally, these highly specific methods cannot be combined, and the results are incomparable. Initially developed for single-molecule DNA sequencing, nanopore-type technologies are expected to enable simultaneous quantitation and location of DNA adducts across the genome. Herein, we briefly summarize the current methodologies for state-of-the-art quantitation of DNA adduct levels and mapping of DNA adducts and describe novel single-molecule DNA sequencing technologies to achieve both measures. Emerging technologies are expected to soon provide a comprehensive picture of the exposome and identify gene regions susceptible to DNA adduct formation.

Phosphorothioate-DNA bacterial diet reduces the ROS levels in C. elegans while improving locomotion and longevity

DNA phosphorothioation (PT) is widely distributed in the human gut microbiome. In this work, PT-diet effect on nematodes was studied with PT-bioengineering bacteria. We found that the ROS level decreased by about 20–50% and the age-related lipofuscin accumulation was reduced by 15–25%. Moreover, the PT-feeding worms were more active at all life periods, and more resistant to acute stressors. Intriguingly, their lifespans were prolonged by ~21.7%. Comparative RNA-seq analysis indicated that many gene expressions were dramatically regulated by PT-diet, such as cysteine-rich protein (scl-11/12/13), sulfur-related enzyme (cpr-2), longevity gene (jnk-1) and stress response (sod-3/5, gps-5/6, gst-18/20, hsp-12.6). Both the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested that neuroactivity pathways were upregulated, while phosphoryl transfer and DNA-repair pathways were down-regulated in good-appetite young worms. The findings pave the way for pro-longevity of multicellular organisms by PT-bacterial interference.

Qiang Huang et al. fed C. elegans with E. coli containing phosphorothioate (PT) DNA or a control strain and evaluated the impact on animal physiology. They observed that worms fed PT( + ) diets exhibited low reactive oxygen species, more active movement, and a longer lifespan compared to controls, suggesting that PT-DNA may have a positive effect on animal health.

The origin and impeded dissemination of the DNA phosphorothioation system in prokaryotes

Phosphorothioate (PT) modification by the dnd gene cluster is the first identified DNA backbone modification and constitute an epigenetic system with multiple functions, including antioxidant ability, restriction modification, and virus resistance. Despite these advantages for hosting dnd systems, they are surprisingly distributed sporadically among contemporary prokaryotic genomes. To address this ecological paradox, we systematically investigate the occurrence and phylogeny of dnd systems, and they are suggested to have originated in ancient Cyanobacteria after the Great Oxygenation Event. Interestingly, the occurrence of dnd systems and prophages is significantly negatively correlated. Further, we experimentally confirm that PT modification activates the filamentous phage SW1 by altering the binding affinity of repressor and the transcription level of its encoding gene. Competition assays, concurrent epigenomic and transcriptomic sequencing subsequently show that PT modification affects the expression of a variety of metabolic genes, which reduces the competitive fitness of the marine bacterium Shewanella piezotolerans WP3. Our findings strongly suggest that a series of negative effects on microorganisms caused by dnd systems limit horizontal gene transfer, thus leading to their sporadic distribution. Overall, our study reveals putative evolutionary scenario of the dnd system and provides novel insights into the physiological and ecological influences of PT modification.

Strategies to Avoid Artifacts in Mass Spectrometry-Based Epitranscriptome Analyses

In this report, we perform structure validation of recently reported RNA phosphorothioate (PT) modifications, a new set of epitranscriptome marks found in bacteria and eukaryotes including humans. By comparing synthetic PT-containing diribonucleotides with native species in RNA hydrolysates by high-resolution mass spectrometry (MS), metabolic stable isotope labeling, and PT-specific iodine-desulfurization, we disprove the existence of PTs in RNA from E. coli, S. cerevisiae, human cell lines, and mouse brain. Furthermore, we discuss how an MS artifact led to the initial misidentification of 2'-O-methylated diribonucleotides as RNA phosphorothioates. To aid structure validation of new nucleic acid modifications, we present a detailed guideline for MS analysis of RNA hydrolysates, emphasizing how the chosen RNA hydrolysis protocol can be a decisive factor in discovering and quantifying RNA modifications in biological samples.

Quantification and mapping of DNA modifications

Apart from the four canonical nucleobases, DNA molecules carry a number of natural modifications. Substantial evidence shows that DNA modifications can regulate diverse biological processes. Dynamic and reversible modifications of DNA are critical for cell differentiation and development. Dysregulation of DNA modifications is closely related to many human diseases. The research of DNA modifications is a rapidly expanding area and has been significantly stimulated by the innovations of analytical methods. With the recent advances in methods and techniques, a series of new DNA modifications have been discovered in the genomes of prokaryotes and eukaryotes. Deciphering the biological roles of DNA modifications depends on the sensitive detection, accurate quantification, and genome-wide mapping of modifications in genomic DNA. This review provides an overview of the recent advances in analytical methods and techniques for both the quantification and genome-wide mapping of natural DNA modifications. We discuss the principles, advantages, and limitations of these developed methods. It is anticipated that new methods and techniques will resolve the current challenges in this burgeoning research field and expedite the elucidation of the functions of DNA modifications.

Site-Specific Scissors Based on Myeloperoxidase for Phosphorothioate DNA

The tool box of site-specific cleavage for nucleic acid has been an increasingly attractive subject. Especially, the recent emergence of the orthogonally activatable DNA device is closely related to the site-specific scission. However, most of these cleavage strategies are based on exogenous assistance, such as laser irradiation. Endogenous strategies are highly desirable for the orthogonally regulatable DNA machine to explore the crucial intracellular biological process and cell signal network. Here, we found that the accurate site-specific cleavage reaction of phosphorothioate (PT) modified DNA by using myeloperoxidase (MPO). A scissors-like mechanism by which MPO breaks PT modification through chloride oxidation has been revealed. Furthermore, we have successfully applied the scissors to activate PT-modified hairpin-DNA machines to produce horseradish peroxidase (HRP)-mimicking DNAzyme or initiate hybridization chain reaction (HCR) amplification. Since MPO plays an important role in the pathway related to oxidative stress in cells, through the HCR amplification activated by this tool box, the oxidative stress in living cells has been robustly imaged. This work proposes an accurate and endogenous site-specific cleavage tool for the research of biostimuli and the construction of DNA molecular devices.

Single-molecule optical mapping of the distribution of DNA phosphorothioate epigenetics

DNA phosphorothioate (PT) modifications, with the nonbridging phosphate oxygen replaced by sulfur, governed by DndABCDE or SspABCD, are widely distributed in prokaryotes and have a highly unusual feature of occupying only a small portion of available consensus sequences in a genome. Despite the presence of plentiful non-PT-protected consensuses, DNA PT modification is still employed as a recognition tag by the restriction cognate, for example, DndFGH or SspE, to discriminate and destroy PT-lacking foreign DNA. This raises a fundamental question about how PT modifications are distributed along DNA molecules to keep the restriction components in check. Here, we present two single-molecule strategies that take advantage of the nucleophilicity of PT in combination with fluorescent markers for optical mapping of both single- and double-stranded PT modifications across individual DNA molecules. Surprisingly, PT profiles vary markedly from molecule to molecule, with different PT locations and spacing distances between PT pairs, even in the presence of DndFGH or SspE. The results revealed unprecedented PT modification features previously obscured by ensemble averaging, providing novel insights into the riddles regarding unusual target selection by PT modification and restriction components.

SspABCD-SspFGH Constitutes a New Type of DNA Phosphorothioate-Based Bacterial Defense System

We recently found that SspABCD, catalyzing single-stranded (ss) DNA phosphorothioate (PT) modification, coupled with SspE provides protection against phage infection. SspE performs both PT-simulated NTPase and DNA-nicking nuclease activities to damage phage DNA, rendering SspA-E a PT-sensing defense system.

Unlike nucleobase modifications in canonical restriction-modification systems, DNA phosphorothioate (PT) epigenetic modification occurs in the DNA sugar-phosphate backbone when the nonbridging oxygen is replaced by sulfur in a double-stranded (ds) or single-stranded (ss) manner governed by DndABCDE or SspABCD, respectively. SspABCD coupled with SspE constitutes a defense barrier in which SspE depends on sequence-specific PT modifications to exert its antiphage activity. Here, we identified a new type of ssDNA PT-based SspABCD-SspFGH defense system capable of providing protection against phages through a mode of action different from that of SspABCD-SspE. We provide further evidence that SspFGH damages non-PT-modified DNA and exerts antiphage activity by suppressing phage DNA replication. Despite their different defense mechanisms, SspFGH and SspE are compatible and pair simultaneously with one SspABCD module, greatly enhancing the protection against phages. Together with the observation that the sspBCD-sspFGH cassette is widely distributed in bacterial genomes, this study highlights the diversity of PT-based defense barriers and expands our knowledge of the arsenal of phage defense mechanisms.

Development of Methods Derived from Iodine-Induced Specific Cleavage for Identification and Quantitation of DNA Phosphorothioate Modifications

DNA phosphorothioate (PT) modification is a novel modification that occurs on the DNA backbone, which refers to a non-bridging phosphate oxygen replaced by sulfur. This exclusive DNA modification widely distributes in bacteria but has not been found in eukaryotes to date. PT modification renders DNA nuclease tolerance and serves as a constitute element of bacterial restriction-modification (R-M) defensive system and more biological functions are awaiting exploration. Identification and quantification of the bacterial PT modifications are thus critical to better understanding their biological functions. This work describes three detailed methods derived from iodine-induced specific cleavage-an iodine-induced cleavage assay (ICA), a deep sequencing of iodine-induced cleavage at PT site (ICDS) and an iodine-induced cleavage PT sequencing (PT-IC-Seq)-for the investigation of PT modifications. Using these approaches, we have identified the presence of PT modifications and quantized the frequency of PT modifications in bacteria. These characterizations contributed to the high-resolution genomic mapping of PT modifications, in which the distribution of PT modification sites on the genome was marked accurately and the frequency of the specific modified sites was reliably obtained. Here, we provide time-saving and less labor-consuming methods for both of qualitative and quantitative analysis of genomic PT modifications. The application of these methodologies will offer great potential for better understanding the biology of the PT modifications and open the door to future further systematical study.

DNA backbone interactions impact the sequence specificity of DNA sulfur-binding domains: revelations from structural analyses

The sulfur atom of phosphorothioated DNA (PT-DNA) is coordinated by a surface cavity in the conserved sulfur-binding domain (SBD) of type IV restriction enzymes. However, some SBDs cannot recognize the sulfur atom in some sequence contexts. To illustrate the structural determinants for sequence specificity, we resolved the structure of SBD_Spr, from endonuclease SprMcrA, in complex with DNA of G_PSGCC, G_PSATC and G_PSAAC contexts. Structural and computational analyses explained why it binds the above PT-DNAs with an affinity in a decreasing order. The structural analysis of SBD_Spr–G_PSGCC and SBD_Sco–G_PSGCC, the latter only recognizes DNA of G_PSGCC, revealed that a positively charged loop above the sulfur-coordination cavity electrostatically interacts with the neighboring DNA phosphate linkage. The structural analysis indicated that the DNA–protein hydrogen bonding pattern and weak non-bonded interaction played important roles in sequence specificity of SBD protein. Exchanges of the positively-charged amino acid residues with the negatively-charged residues in the loop would enable SBD_Sco to extend recognization for more PT-DNA sequences, implying that type IV endonucleases can be engineered to recognize PT-DNA in novel target sequences.

Defense Mechanism of Phosphorothioated DNA under Peroxynitrite-Mediated Oxidative Stress

DNA phosphorothioation (PT) exists in many pathogenic bacteria; however, the mechanism of PT-DNA resistance to the immune response is unclear. In this work, we meticulously investigated the peroxynitrite (PN) tolerance using PT-bioengineered E. coli strains. The in vivo experiment confirms that the S⁺ strain survives better than the S^- strain under moderately oxidative stress. The LCMS, IC, and GCMS experiments demonstrated that phosphorothioate partially converted to phosphate, and the byproduct included sulfate and elemental sulfur. When O,O-diethyl thiophosphate ester (DETP) was used, the reaction rate k₁ was determined to be 4.3 ± 0.5 M^-1 s^-1 in the first-order for both phosphorothioate and peroxynitrite at 35 °C and pH of 8.0. The IC₅₀ values of phosphorothioate dinucleotides are dramatically increased by 400-700-fold compared to DETP. The SH/OH Yin-Yang mechanism rationalizes the in situ DNA self-defense against PN-mediated oxidative stress at the extra bioenergetic cost of DNA modification.

DNA Phosphorothioate Modifications Are Widely Distributed in the Human Microbiome

The DNA phosphorothioate (PT) modification existing in many prokaryotes, including bacterial pathogens and commensals, confers multiple characteristics, including restricting gene transfer, influencing the global transcriptional response, and reducing fitness during exposure to chemical mediators of inflammation. While PT-containing bacteria have been investigated in a variety of environments, they have not been studied in the human microbiome. Here, we investigated the distribution of PT-harboring strains and verified their existence in the human microbiome. We found over 2000 PT gene-containing strains distributed in different body sites, especially in the gastrointestinal tract. PT-modifying genes are preferentially distributed within several genera, including Pseudomonas, Clostridioides, and Escherichia, with phylogenic diversities. We also assessed the PT modification patterns and found six new PT-linked dinucleotides (C_psG, C_psT, A_psG, T_psG, G_psC, A_psT) in human fecal DNA. To further investigate the PT in the human gut microbiome, we analyzed the abundance of PT-modifying genes and quantified the PT-linked dinucleotides in the fecal DNA. These results confirmed that human microbiome is a rich reservoir for PT-containing microbes and contains a wide variety of PT modification patterns.

Nick-seq for single-nucleotide resolution genomic maps of DNA modifications and damage

DNA damage and epigenetic marks are well established to have profound influences on genome stability and cell phenotype, yet there are few technologies to obtain high-resolution genomic maps of the many types of chemical modifications of DNA. Here we present Nick-seq for quantitative, sensitive, and accurate mapping of DNA modifications at single-nucleotide resolution across genomes. Pre-existing breaks are first blocked and DNA modifications are then converted enzymatically or chemically to strand-breaks for both 3'-extension by nick-translation to produce nuclease-resistant oligonucleotides and 3'-terminal transferase tailing. Following library preparation and next generation sequencing, the complementary datasets are mined with a custom workflow to increase sensitivity, specificity and accuracy of the map. The utility of Nick-seq is demonstrated with genomic maps of site-specific endonuclease strand-breaks in purified DNA from Eschericia coli, phosphorothioate epigenetics in Salmonella enterica Cerro 87, and oxidation-induced abasic sites in DNA from E. coli treated with a sublethal dose of hydrogen peroxide. Nick-seq applicability is demonstrated with strategies for >25 types of DNA modification and damage.

Epigenetic competition reveals density-dependent regulation and target site plasticity of phosphorothioate epigenetics in bacteria

Phosphorothioate (PT) DNA modifications-in which a nonbonding phosphate oxygen is replaced with sulfur-represent a widespread, horizontally transferred epigenetic system in prokaryotes and have a highly unusual property of occupying only a small fraction of available consensus sequences in a genome. Using Salmonella enterica as a model, we asked a question of fundamental importance: How do the PT-modifying DndA-E proteins select their G_PSAAC/G_PSTTC targets? Here, we applied innovative analytical, sequencing, and computational tools to discover a novel behavior for DNA-binding proteins: The Dnd proteins are "parked" at the G^6mATC Dam methyltransferase consensus sequence instead of the expected GAAC/GTTC motif, with removal of the ^6mA permitting extensive PT modification of GATC sites. This shift in modification sites further revealed a surprising constancy in the density of PT modifications across the genome. Computational analysis showed that GAAC, GTTC, and GATC share common features of DNA shape, which suggests that PT epigenetics are regulated in a density-dependent manner partly by DNA shape-driven target selection in the genome.

Structural Analysis of an l-Cysteine Desulfurase from an Ssp DNA Phosphorothioation System

Apart from its roles in Fe-S cluster assembly, tRNA thiolation, and sulfur-containing cofactor biosynthesis, cysteine desulfurase serves as a sulfur donor in the DNA PT modification, in which a sulfur atom substitutes a nonbridging oxygen in the DNA phosphodiester backbone. The initial sulfur mobilization from l-cysteine is catalyzed by the SspA cysteine desulfurase in the SspABCD-mediated DNA PT modification system. By determining the crystal structure of SspA, the study presents the molecular mechanism that SspA employs to recognize its cysteine substrate and PLP cofactor. To overcome the long distance (8.9 Å) between the catalytic Cys314 and the cysteine substrate, a conformational change occurs to bring Cys314 to the vicinity of the substrate, allowing for nucleophilic attack.

DNA phosphorothioate (PT) modification, in which the nonbridging oxygen in the sugar-phosphate backbone is substituted by sulfur, is catalyzed by DndABCDE or SspABCD in a double-stranded or single-stranded manner, respectively. In Dnd and Ssp systems, mobilization of sulfur in PT formation starts with the activation of the sulfur atom of cysteine catalyzed by the DndA and SspA cysteine desulfurases, respectively. Despite playing the same biochemical role, SspA cannot be functionally replaced by DndA, indicating its unique physiological properties. In this study, we solved the crystal structure of Vibrio cyclitrophicus SspA in complex with its natural substrate, cysteine, and cofactor, pyridoxal phosphate (PLP), at a resolution of 1.80 Å. Our solved structure revealed the molecular mechanism that SspA employs to recognize its cysteine substrate and PLP cofactor, suggesting a common binding mode shared by cysteine desulfurases. In addition, although the distance between the catalytic Cys314 and the substrate cysteine is 8.9 Å, which is too far for direct interaction, our structural modeling and biochemical analysis revealed a conformational change in the active site region toward the cysteine substrate to move them close to each other to facilitate the nucleophilic attack. Finally, the pulldown analysis showed that SspA could form a complex with SspD, an ATP pyrophosphatase, suggesting that SspD might potentially accept the activated sulfur atom directly from SspA, providing further insights into the biochemical pathway of Ssp-mediated PT modification.

SspABCD-SspE is a phosphorothioation-sensing bacterial defence system with broad anti-phage activities

Bacteria have evolved diverse mechanisms to fend off predation by bacteriophages. We previously identified the Dnd system, which uses DndABCDE to insert sulfur into the DNA backbone as a double-stranded phosphorothioate (PT) modification, and DndFGH, a restriction component. Here, we describe an unusual SspABCD-SspE PT system in Vibrio cyclitrophicus, Escherichia coli and Streptomyces yokosukanensis, which has distinct genetic organization, biochemical functions and phenotypic behaviour. SspABCD confers single-stranded and high-frequency PTs with SspB acting as a nickase and possibly introducing nicks to facilitate sulfur incorporation. Strikingly, SspABCD coupled with SspE provides protection against phages in unusual ways: (1) SspE senses sequence-specific PTs by virtue of its PT-stimulated NTPase activity to exert its anti-phage activity, and (2) SspE inhibits phage propagation by introducing nicking damage to impair phage DNA replication. These results not only expand our knowledge about the diversity and functions of DNA PT modification but also enhance our understanding of the known arsenal of defence systems.

An in vitro DNA phosphorothioate modification reaction

Phosphorothioation (PT) involves the replacement of a nonbridging phosphate oxygen on the DNA backbone with sulfur. In bacteria, the procedure is both sequence- and stereo-specific. We reconstituted the PT reaction using purified DndCDE from Salmonella enterica and IscS from Escherichia coli. We determined that the in vitro process of PT was oxygen sensitive. Only one strand on a double-stranded (ds) DNA substrate was modified in the reaction. The modification was dominant between G and A in the GAAC/GTTC conserved sequence. The modification between G and T required the presence of PT between G and A on the opposite strand. Cysteine, S-adenosyl methionine (SAM) and the formation of an iron-sulfur cluster in DndCDE (DndCDE-FeS) were essential for the process. Results from SAM cleavage reactions support the supposition that PT is a radical SAM reaction. Adenosine triphosphate (ATP) promoted the reaction but was not essential. The data and conclusions presented suggest that the PT reaction in bacteria involves three steps. The first step is the binding of DndCDE-FeS to DNA and searching for the modification sequence, possibly with the help of ATP. Cysteine locks DndCDE-FeS to the modification site with an appropriate protein conformation. SAM triggers the radical SAM reaction to complete the oxygen-sulfur swapping.

The Biological Applications of Two Aggregation-Induced Emission Luminogens

Fluorescence imaging, as a commonly used scientific tool, is widely applied in various biomedical and material structures through visualization technology. Highly selective and sensitive luminescent biological probes, as well as those with good water solubility, are urgently needed for biomedical research. In contrast to the traditional aggregation-caused quenching of fluorescence, in the unique phenomenon of aggregation-induced emission (AIE), the individual luminogens have extremely weak or no emissivity because they each have free intramolecular motion; however, when they form aggregates, these components immediately "light up". Since the discovery of "turn-on" mechanism, researchers have been studying and applying AIE in a variety of fields to develop more sensitive, selective, and efficient strategies for the AIE dyes. There are numerous advantages to the use of AIE-based methods, including low background interference, strong contrast, high performance in intracellular imaging, and the ability for long-term monitoring in vivo. In this review, two typical examples of AIEgens, TPE-Cy and TPE-Ph-In, are described, including their structure properties and applications. Recent progress in the biological applications is mainly focused on. Undoubtedly, in the near future, an increasing number of encouraging and practical ideas will promote the development of more AIEgens for broad use in biomedical applications.

Shewanella decolorationis LDS1 Chromate Resistance

The genus Shewanella is well known for its genetic diversity, its outstanding respiratory capacity, and its high potential for bioremediation. Here, a novel strain isolated from sediments of the Indian Ocean was characterized. A 16S rRNA analysis indicated that it belongs to the species Shewanella decolorationis It was named Shewanella decolorationis LDS1. This strain presented an unusual ability to grow efficiently at temperatures from 24°C to 40°C without apparent modifications of its metabolism, as shown by testing respiratory activities or carbon assimilation, and in a wide range of salt concentrations. Moreover, S. decolorationis LDS1 tolerates high chromate concentrations. Indeed, it was able to grow in the presence of 4 mM chromate at 28°C and 3 mM chromate at 40°C. Interestingly, whatever the temperature, when the culture reached the stationary phase, the strain reduced the chromate present in the growth medium. In addition, S. decolorationis LDS1 degrades different toxic dyes, including anthraquinone, triarylmethane, and azo dyes. Thus, compared to Shewanella oneidensis, this strain presented better capacity to cope with various abiotic stresses, particularly at high temperatures. The analysis of genome sequence preliminary data indicated that, in contrast to S. oneidensis and S. decolorationis S12, S. decolorationis LDS1 possesses the phosphorothioate modification machinery that has been described as participating in survival against various abiotic stresses by protecting DNA. We demonstrate that its heterologous production in S. oneidensis allows it to resist higher concentrations of chromate.IMPORTANCE Shewanella species have long been described as interesting microorganisms in regard to their ability to reduce many organic and inorganic compounds, including metals. However, members of the Shewanella genus are often depicted as cold-water microorganisms, although their optimal growth temperature usually ranges from 25 to 28°C under laboratory growth conditions. Shewanella decolorationis LDS1 is highly attractive, since its metabolism allows it to develop efficiently at temperatures from 24 to 40°C, conserving its ability to respire alternative substrates and to reduce toxic compounds such as chromate or toxic dyes. Our results clearly indicate that this novel strain has the potential to be a powerful tool for bioremediation and unveil one of the mechanisms involved in its chromate resistance.

Phosphorothioation of foreign DNA influences the transformation efficiency in Clostridium perfringens NCTC8239

Major advances in Clostridium perfringens genetics have been achieved through the development of electroporation-induced transformation; however, highly transformable strains are still limited. SM101 is the only useful strain for genetic manipulation via transformation in C. perfringens causing foodborne illness (FBI). We focused on the FBI strain NCTC8239, which is transformed at a low frequency, because it has a gene cassette that is predicted to encode enzymes involved in DNA phosphorothioation (PT). The oxidant-dependent degradation of NCTC8239 genomic DNA suggested that the genome is PT-modified. When foreign DNA was PT-modified using a plasmid expressing Salmonella enterica PT modification enzymes, the transformation efficiency of NCTC8239 was significantly higher than that using an unmodified plasmid. We then attempted to establish a highly transformable derivative of NCTC8239, and focused on DptFGH, which are predicted to be PT restriction enzymes. A dptG-null mutant exhibited significantly higher transformation efficiency with unmodified foreign DNA than did the wild-type strain. Furthermore, the mutant was transformed with the unmodified plasmid as efficiently as with a PT-modified plasmid, implying that DptG is involved in PT-dependent restriction. Thus, the present results revealed that PT modifications of foreign DNA influence the transformation frequency of NCTC8239 and suggest that PT is a factor contributing to transformation efficiency in NCTC8239.

Novel Iodine-induced Cleavage Real-time PCR Assay for Accurate Quantification of Phosphorothioate Modified Sites in Bacterial DNA

DNA Phosphorothioate (PT), replacing a non-bridging phosphate oxygen atom with a sulfur atom, is one kind of common DNA modification in bacteria. Whole genome scale description of the location and frequency of PT modification is the key to understand its biological function. Herein we developed a novel method, named with iodine-induced cleavage quantitative real-time PCR (IC-qPCR), to evaluate the frequency of PT modification at a given site in bacterial DNA. The efficiency, dynamic range, sensitivity, reproducibility and accuracy of IC-qPCR were well tested and verified employing an E. coli B7A strain as example. The amplification efficiency of IC-qPCR assay ranged from 91% to 99% with a high correlation coefficient ≥0.99. The limit of quantification was determined as low as 10 copies per reaction for the 607710 and 1818096 sites, and 5 copies for the 302695 and 4120753 sites. Based on the developed IC-qPCR method, the modification frequency of four PTs in E. coli B7A was determined with high accuracy, and the results showed that the PT modification was partial and that the modification frequency varied among investigated PT sites. All these results showed that IC-qPCR was suitable for evaluating the PT modification, which would be helpful to further understand the biological function of PT modification.

DNA phosphorothioate modification-a new multi-functional epigenetic system in bacteria

Synthetic phosphorothioate (PT) internucleotide linkages, in which a nonbridging oxygen is replaced by a sulphur atom, share similar physical and chemical properties with phosphodiesters but confer enhanced nuclease tolerance on DNA/RNA, making PTs a valuable biochemical and pharmacological tool. Interestingly, PT modification was recently found to occur naturally in bacteria in a sequence-selective and RP configuration-specific manner. This oxygen-sulphur swap is catalysed by the gene products of dndABCDE, which constitute a defence barrier with DndFGH in some bacterial strains that can distinguish and attack non-PT-modified foreign DNA, resembling DNA methylation-based restriction-modification (R-M) systems. Despite their similar defensive mechanisms, PT- and methylation-based R-M systems have evolved to target different consensus contexts in the host cell because when they share the same recognition sequences, the protective function of each can be impeded. The redox and nucleophilic properties of PT sulphur render PT modification a versatile player in the maintenance of cellular redox homeostasis, epigenetic regulation and environmental fitness. The widespread presence of dnd systems is considered a consequence of extensive horizontal gene transfer, whereas the lability of PT during oxidative stress and the susceptibility of PT to PT-dependent endonucleases provide possible explanations for the ubiquitous but sporadic distribution of PT modification in the bacterial world.

Gut microbiome interventions in human health and diseases

Ongoing studies have determined that the gut microbiota is a major factor influencing both health and disease. Host genetic factors and environmental factors contribute to differences in gut microbiota composition and function. Intestinal dysbiosis is a cause or a contributory cause for diseases in multiple body systems, ranging from the digestive system to the immune, cardiovascular, respiratory, and even nervous system. Investigation of pathogenesis has identified specific species or strains, bacterial genes, and metabolites that play roles in certain diseases and represent potential drug targets. As research progresses, gut microbiome-based diagnosis and therapy are proposed and applied, which might lead to considerable progress in precision medicine. We further discuss the limitations of current studies and potential solutions.

A new type of DNA phosphorothioation-based antiviral system in archaea

Archaea and Bacteria have evolved different defence strategies that target virtually all steps of the viral life cycle. The diversified virion morphotypes and genome contents of archaeal viruses result in a highly complex array of archaea-virus interactions. However, our understanding of archaeal antiviral activities lags far behind our knowledges of those in bacteria. Here we report a new archaeal defence system that involves DndCDEA-specific DNA phosphorothioate (PT) modification and the PbeABCD-mediated halt of virus propagation via inhibition of DNA replication. In contrast to the breakage of invasive DNA by DndFGH in bacteria, DndCDEA-PbeABCD does not degrade or cleave viral DNA. The PbeABCD-mediated PT defence system is widespread and exhibits extensive interdomain and intradomain gene transfer events. Our results suggest that DndCDEA-PbeABCD is a new type of PT-based virus resistance system, expanding the known arsenal of defence systems as well as our understanding of host-virus interactions.

Phosphorothioated DNA Is Shielded from Oxidative Damage

DNA is the carrier of genetic information. DNA modifications play a central role in essential physiological processes. Phosphorothioation (PT) modification involves the replacement of an oxygen atom on the DNA backbone with a sulfur atom. PT modification can cause genomic instability in Salmonella enterica under hypochlorous acid stress. This modification restores hydrogen peroxide (H₂O₂) resistance in the catalase-deficient Escherichia coli Hpx^- strain. Here, we report biochemical characterization results for a purified PT modification protein complex (DndCDE) from S. enterica We observed multiplex oligomeric states of DndCDE by using native PAGE. This protein complex bound avidly to PT-modified DNA. DndCDE with an intact iron-sulfur cluster (DndCDE-FeS) possessed H₂O₂ decomposition activity, with a V _max of 10.58 ± 0.90 mM min^-1 and a half-saturation constant, K _0.5S, of 31.03 mM. The Hill coefficient was 2.419 ± 0.59 for this activity. The protein's activity toward H₂O₂ was observed to be dependent on the intact DndCDE and on the formation of an iron-sulfur (Fe-S) cluster on the DndC subunit. In addition to cysteine residues that mediate the formation of this Fe-S cluster, other cysteine residues play a catalytic role. Finally, catalase activity was also detected in DndCDE from Pseudomonas fluorescens Pf0-1. The data and conclusions presented suggest that DndCDE-FeS is a short-lived catalase. Our experiments also indicate that the complex binds to PT sites, shielding PT DNA from H₂O₂ damage. This catalase shield might be able to extend from PT sites to the entire bacterial genome.IMPORTANCE DNA phosphorothioation has been reported in many bacteria. These PT-hosting bacteria live in very different environments, such as the human body, soil, or hot springs. The physiological function of DNA PT modification is still elusive. A remarkable property of PT modification is that purified genomic PT DNA is susceptible to oxidative cleavage. Among the oxidants, hypochlorous acid and H₂O₂ are of physiological relevance for human pathogens since they are generated during the human inflammation response to bacterial infection. However, expression of PT genes in the catalase-deficient E. coli Hpx^- strain restores H₂O₂ resistance. Here, we seek to solve this obvious paradox. We demonstrate that DndCDE-FeS is a short-lived catalase that binds tightly to PT DNA. It is thus possible that by docking to PT sites the catalase activity protects the bacterial genome against H₂O₂ damage.

Quantitative mapping of DNA phosphorothioatome reveals phosphorothioate heterogeneity of low modification frequency

Phosphorothioate (PT) modifications of the DNA backbone, widespread in prokaryotes, are first identified in bacterial enteropathogens Escherichia coli B7A more than a decade ago. However, methods for high resolution mapping of PT modification level are still lacking. Here, we developed the PT-IC-seq technique, based on iodine-induced selective cleavage at PT sites and high-throughput next generation sequencing, as a mean to quantitatively characterizing the genomic landscape of PT modifications. Using PT-IC-seq we foud that most PT sites are partially modified at a lower PT frequency (< 5%) in E. coli B7A and Salmonella enterica serovar Cerro 87, and both show a heterogeneity pattern of PT modification similar to those of the typical methylation modification. Combining the iodine-induced cleavage and absolute quantification by droplet digital PCR, we developed the PT-IC-ddPCR technique to further measure the PT modification level. Consistent with the PT-IC-seq measurements, PT-IC-ddPCR analysis confirmed the lower PT frequency in E. coli B7A. Our study has demonstrated the heterogeneity of PT modification in the bacterial population and we also established general tools for rigorous mapping and characterization of PT modification events at whole genome level. We describe to our knowledge the first genome-wide quantitative characterization of PT landscape and provides appropriate strategies for further functional studies of PT modification.

Advances in CRISPR-Cas systems for RNA targeting, tracking and editing

Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) systems, especially type II (Cas9) systems, have been widely used in gene/genome targeting. Modifications of Cas9 enable these systems to become platforms for precise DNA manipulations. However, the utilization of CRISPR-Cas systems in RNA targeting remains preliminary. The discovery of type VI CRISPR-Cas systems (Cas13) shed light on RNA-guided RNA targeting. Cas13d, the smallest Cas13 protein, with a length of only ~930 amino acids, is a promising platform for RNA targeting compatible with viral delivery systems. Much effort has also been made to develop Cas9, Cas13a and Cas13b applications for RNA-guided RNA targeting. The discovery of new RNA-targeting CRISPR-Cas systems as well as the development of RNA-targeting platforms with Cas9 and Cas13 will promote RNA-targeting technology substantially. Here, we review new advances in RNA-targeting CRISPR-Cas systems as well as advances in applications of these systems in RNA targeting, tracking and editing. We also compare these Cas protein-based technologies with traditional technologies for RNA targeting, tracking and editing. Finally, we discuss remaining questions and prospects for the future.