Pathological phenotypes and in vivo DNA cleavage by unrestrained activity of a phosphorothioate-based restriction system in Salmonella

The Restriction Activity Investigation of Rv2528c, an Mrr-like Modification-Dependent Restriction Endonuclease from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb), as a typical intracellular pathogen, possesses several putative restriction-modification (R-M) systems, which restrict exogenous DNA's entry, such as bacterial phage infection. Here, we investigate Rv2528c, a putative Mrr-like type IV restriction endonuclease (REase) from Mtb H37Rv, which is predicted to degrade methylated DNA that contains m6A, m5C, etc. Rv2528c shows significant cytotoxicity after being expressed in Escherichia coli BL21(DE3)pLysS strain. The Terminal deoxynucleotidyl transferase dUTP Nick-End Labeling (TUNEL) assay indicates that Rv2528c cleaves genomic DNA in vivo. The plasmid transformation efficiency of BL21(DE3)pLysS strain harboring Rv2528c gene was obviously decreased after plasmids were in vitro methylated by commercial DNA methyltransferases such as M.EcoGII, M.HhaI, etc. These results are consistent with the characteristics of type IV REases. The in vitro DNA cleavage condition and the consensus cleavage/recognition site of Rv2528c still remain unclear, similar to that of most Mrr-family proteins. The possible reasons mentioned above and the potential role of Rv2528c for Mtb were discussed.

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.

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.

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.

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.

Microbial Arsenal of Antiviral Defenses - Part I

Bacteriophages or phages are viruses that infect bacterial cells (for the scope of this review we will also consider viruses that infect Archaea). Constant threat of phage infection is a major force that shapes evolution of the microbial genomes. To withstand infection, bacteria had evolved numerous strategies to avoid recognition by phages or to directly interfere with phage propagation inside the cell. Classical molecular biology and genetic engineering have been deeply intertwined with the study of phages and host defenses. Nowadays, owing to the rise of phage therapy, broad application of CRISPR-Cas technologies, and development of bioinformatics approaches that facilitate discovery of new systems, phage biology experiences a revival. This review describes variety of strategies employed by microbes to counter phage infection, with a focus on novel systems discovered in recent years. First chapter covers defense associated with cell surface, role of small molecules, and innate immunity systems relying on DNA modification.

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.

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.

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.

Investigating the interactions between DNA and DndE during DNA phosphorothioation

The DNA phosphorothioate modification is a novel physiological variation in bacteria. DndE controls this modification by binding to dsDNA via a mechanism that remains unclear. Structural analysis of the wild-type DndE tetramer suggests that a positively charged region in its center is important for DNA binding. In the present study, we replaced residues G21 and G24 in this region with lysines, which increases the DNA binding affinity but does not affect the DNA degradation phenotype. Structural analysis of the mutant indicates that it forms a new tetrameric conformation and that DndE interacts with DNA as a monomer rather than as a tetramer. A structural model of the DndE-DNA complex, based on its structural homolog P22 Arc repressor, indicates that two flexible loops in DndE are determinants of DNA binding.

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.

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.

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.

Tight control of genomic phosphorothioate modification by the ATP-modulated autoregulation and reusability of DndB

DNA phosphorothioate (PT) modification was recently identified to occur naturally in diverse bacteria and to be governed by DndABCDE proteins. The nuclease resistance as well as the redox and nucleophilic properties of PT sulfur make PT modification a versatile player in restriction-modification (R-M) defense, epigenetic regulation, environmental fitness and the maintenance of cellular redox homeostasis. In this study, we discovered that tight control of PT levels is mediated by the ATPase activity of DndB. The ATP-binding activity of DndB stimulates the dissociation of the DndB-DNA complex, allowing transcriptional initiation, whereas its ATP hydrolysis activity promotes the conversion of DndB-ATP to free DndB that is capable of rebinding to promoter DNA for transcriptional inhibition. Since sulfur incorporation is an ATP-consuming process, these activities provide an economical way to fine-tune PT modification in an ATP-sensing manner. To our knowledge, this ATP-mediated regulation is a rare example among DNA epigenetic modification systems; the features of autoregulation and the repeated usage of DndB allow the dedicated regulation of PT levels in response to cellular ATP concentrations, providing insight into PT function and its role in physiology.

Structural basis for the recognition of sulfur in phosphorothioated DNA

There have been very few reports on protein domains that specifically recognize sulfur. Here we present the crystal structure of the sulfur-binding domain (SBD) from the DNA phosphorothioation (PT)-dependent restriction endonuclease ScoMcrA. SBD contains a hydrophobic surface cavity that is formed by the aromatic ring of Y164, the pyrolidine ring of P165, and the non-polar side chains of four other residues that serve as lid, base, and wall of the cavity. The SBD and PT-DNA undergo conformational changes upon binding. The S¹⁸⁷RGRR¹⁹¹ loop inserts into the DNA major groove to make contacts with the bases of the G_PSGCC core sequence. Mutating key residues of SBD impairs PT-DNA association. More than 1000 sequenced microbial species from fourteen phyla contain SBD homologs. We show that three of these homologs bind PT-DNA in vitro and restrict PT-DNA gene transfer in vivo. These results show that SBD-like PT-DNA readers exist widely in prokaryotes.

Regulation of dndB Gene Expression in Streptomyces lividans

DNA sulfur modification is a unique modification occurring in the sugar-phosphate backbone of DNA, with a nonbridging oxygen atom substituted with sulfur in a sequence-specific and Rp stereo-specific manner. Bioinformatics, RNA-seq, and in vitro transcriptional analyses have shown that DNA sulfur modification may be involved in epigenetic regulation. However, the in vivo evidence supporting this assertion is not convincing. Here, we aimed to characterize two sulfur-modified sites near the dndB promoter region in Streptomyces lividans. Single mutation of either site had no effect on dndB transcription, whereas double mutation of both sites significantly elevated dndB expression. These findings suggested that DNA sulfur modification affected gene expression, and the role of DNA sulfur modification in epigenetic regulation depended on the number of sulfur-modified sites. We also identified an inverted repeat, the R repeat sequence, and showed that this sequence participated in the positive regulation of dndB gene expression.

Occurrence, evolution, and functions of DNA phosphorothioate epigenetics in bacteria

The chemical diversity of physiological DNA modifications has expanded with the identification of phosphorothioate (PT) modification in which the nonbridging oxygen in the sugar-phosphate backbone of DNA is replaced by sulfur. Together with DndFGH as cognate restriction enzymes, DNA PT modification, which is catalyzed by the DndABCDE proteins, functions as a bacterial restriction-modification (R-M) system that protects cells against invading foreign DNA. However, the occurrence of dnd systems across a large number of bacterial genomes and their functions other than R-M are poorly understood. Here, a genomic survey revealed the prevalence of bacterial dnd systems: 1,349 bacterial dnd systems were observed to occur sporadically across diverse phylogenetic groups, and nearly half of these occur in the form of a solitary dndBCDE gene cluster that lacks the dndFGH restriction counterparts. A phylogenetic analysis of 734 complete PT R-M pairs revealed the coevolution of M and R components, despite the observation that several PT R-M pairs appeared to be assembled from M and R parts acquired from distantly related organisms. Concurrent epigenomic analysis, transcriptome analysis, and metabolome characterization showed that a solitary PT modification contributed to the overall cellular redox state, the loss of which perturbed the cellular redox balance and induced Pseudomonas fluorescens to reconfigure its metabolism to fend off oxidative stress. An in vitro transcriptional assay revealed altered transcriptional efficiency in the presence of PT DNA modification, implicating its function in epigenetic regulation. These data suggest the versatility of PT in addition to its involvement in R-M protection.

Oxidation of phosphorothioate DNA modifications leads to lethal genomic instability

Genomic modification with sulfur as phosphorothioate (PT) is widespread among prokaryotes, including human pathogens. Apart from its physiological functions, the redox and nucleophilic properties of PT sulfur suggest effects on bacterial fitness in stressful environments. Here we show that PTs are dynamic and labile DNA modifications that cause genomic instability during oxidative stress. Using coupled isotopic labeling-mass spectrometry, we observed sulfur replacement in PTs at a rate of ~2%/h in unstressed Escherichia coli and Salmonella enterica. While PT levels were unaffected by exposure to hydrogen peroxide (H₂O₂) or hypochlorous acid (HOCl), PT turnover increased to 3.8–10%/h for HOCl and was unchanged for H₂O₂, consistent with repair of HOCl-induced sulfur damage. PT-dependent HOCl sensitivity extended to cytotoxicity and DNA strand-breaks, which occurred at orders-of-magnitude lower doses of HOCl than H₂O₂. The genotoxicity of HOCl in PT-containing bacteria suggests reduced fitness in competition with HOCl-producing organisms and during human infections.

Convergence of DNA methylation and phosphorothioation epigenetics in bacterial genomes

Explosive growth in the study of microbial epigenetics has revealed a diversity of chemical structures and biological functions of DNA modifications in restriction-modification (R-M) and basic genetic processes. Here, we describe the discovery of shared consensus sequences for two seemingly unrelated DNA modification systems, ^6mA methylation and phosphorothioation (PT), in which sulfur replaces a nonbridging oxygen in the DNA backbone. Mass spectrometric analysis of DNA from Escherichia coli B7A and Salmonella enterica serovar Cerro 87, strains possessing PT-based R-M genes, revealed d(G_PS^6mA) dinucleotides in the G_PS^6mAAC consensus representing ∼5% of the 1,100 to 1,300 PT-modified d(G_PSA) motifs per genome, with ^6mA arising from a yet-to-be-identified methyltransferase. To further explore PT and ^6mA in another consensus sequence, G_PS^6mATC, we engineered a strain of E. coli HST04 to express Dnd genes from Hahella chejuensis KCTC2396 (PT in G_PSATC) and Dam methyltransferase from E. coli DH10B (^6mA in G^6mATC). Based on this model, in vitro studies revealed reduced Dam activity in G_PSATC-containing oligonucleotides whereas single-molecule real-time sequencing of HST04 DNA revealed ^6mA in all 2,058 G_PSATC sites (5% of 37,698 total GATC sites). This model system also revealed temperature-sensitive restriction by DndFGH in KCTC2396 and B7A, which was exploited to discover that ^6mA can substitute for PT to confer resistance to restriction by the DndFGH system. These results point to complex but unappreciated interactions between DNA modification systems and raise the possibility of coevolution of interacting systems to facilitate the function of each.

Mechanistic Investigation on ROS Resistance of Phosphorothioated DNA

Phosphorothioated DNA (PT-DNA) exhibits a mild anti-oxidant property both in vivo and in vitro. It was found that 8-OHdG and ROS levels were significantly lower in dnd+ (i.e. S⁺) E. coli., compared to a dnd− (i.e. S⁻) strain. Furthermore, different from traditional antioxidants, phosphorothioate compound presents an unexpectedly high capacity to quench hydroxyl radical. Oxidative product analysis by liquid chromatography-mass spectrometry and quantum mechanistic computation supported its unique anti-oxidant characteristic of the hydroxyl selectivity: phosphorothioate donates an electron to either hydroxyl radical or guanine radical derived from hydroxyl radical, leading to a PS^• radical; a complex of PS^• radical and OH⁻ (i.e. the reductive product of hydroxyl radical) releases a highly reductive HS^• radical, which scavenges more equivalents of oxidants in the way to high-covalent sulphur compounds such as sulphur, sulphite and sulphate. The PS-PO conversion (PS and PO denote phosphorus-sulphur and phosphorus-oxygen compounds, respectively) made a switch of extremely oxidative OH^• to highly reductive HS^• species, endowing PT-DNA with the observed high capacity in hydroxyl-radical neutralization. This plausible mechanism provides partial rationale as to why bacteria develop the resource-demanding PT modification on guanine-neighboring phosphates in genome.

DNA Phosphorothioate Modification Plays a Role in Peroxides Resistance in Streptomyces lividans

DNA phosphorothioation, conferred by dnd genes, was originally discovered in the soil-dwelling bacterium Streptomyces lividans, and thereafter found to exist in various bacterial genera. However, the physiological significance of this sulfur modification of the DNA backbone remains unknown in S. lividans. Our studies indicate that DNA phosphorothioation has a major role in resistance to oxidative stress in the strain. Although Streptomyces species express multiple catalase/peroxidase and organic hydroperoxide resistance genes to protect them against peroxide damage, a wild type strain of S. lividans exhibited two-fold to 10-fold higher survival, compared to a dnd⁻ mutant, following treatment with peroxides. RNA-seq experiments revealed that, catalase and organic hydroperoxide resistance gene expression were not up-regulated in the wild type strain, suggesting that the resistance to oxidative stress was not due to the up-regulation of these genes by DNA phosphorothioation. Quantitative RT-PCR analysis was conducted to trace the expression of the catalase and the organic hydroperoxide resistance genes after peroxides treatments. A bunch of these genes were activated in the dnd⁻ mutant rather than the wild type strain in response to peroxides. Moreover, the organic hydroperoxide peracetic acid was scavenged more rapidly in the presence than in the absence of phosphorothioate modification, both in vivo and in vitro. The dnd gene cluster can be up-regulated by the disulfide stressor diamide. Overall, our observations suggest that DNA phosphorothioate modification functions as a peroxide resistance system in S. lividans.

Novel genomic island modifies DNA with 7-deazaguanine derivatives

The discovery of ∼20-kb gene clusters containing a family of paralogs of tRNA guanosine transglycosylase genes, called tgtA5, alongside 7-cyano-7-deazaguanine (preQ0) synthesis and DNA metabolism genes, led to the hypothesis that 7-deazaguanine derivatives are inserted in DNA. This was established by detecting 2'-deoxy-preQ0 and 2'-deoxy-7-amido-7-deazaguanosine in enzymatic hydrolysates of DNA extracted from the pathogenic, Gram-negative bacteria Salmonella enterica serovar Montevideo. These modifications were absent in the closely related S. enterica serovar Typhimurium LT2 and from a mutant of S Montevideo, each lacking the gene cluster. This led us to rename the genes of the S. Montevideo cluster as dpdA-K for 7-deazapurine in DNA. Similar gene clusters were analyzed in ∼150 phylogenetically diverse bacteria, and the modifications were detected in DNA from other organisms containing these clusters, including Kineococcus radiotolerans, Comamonas testosteroni, and Sphingopyxis alaskensis Comparative genomic analysis shows that, in Enterobacteriaceae, the cluster is a genomic island integrated at the leuX locus, and the phylogenetic analysis of the TgtA5 family is consistent with widespread horizontal gene transfer. Comparison of transformation efficiencies of modified or unmodified plasmids into isogenic S. Montevideo strains containing or lacking the cluster strongly suggests a restriction-modification role for the cluster in Enterobacteriaceae. Another preQ0 derivative, 2'-deoxy-7-formamidino-7-deazaguanosine, was found in the Escherichia coli bacteriophage 9 g, as predicted from the presence of homologs of genes involved in the synthesis of the archaeosine tRNA modification. These results illustrate a deep and unexpected evolutionary connection between DNA and tRNA metabolism.

DndEi Exhibits Helicase Activity Essential for DNA Phosphorothioate Modification and ATPase Activity Strongly Stimulated by DNA Substrate with a GAAC/GTTC Motif

Phosphorothioate (PT) modification of DNA, in which the non-bridging oxygen of the backbone phosphate group is replaced by sulfur, is governed by the DndA-E proteins in prokaryotes. To better understand the biochemical mechanism of PT modification, functional analysis of the recently found PT-modifying enzyme DndEi, which has an additional domain compared with canonical DndE, from Riemerella anatipestifer is performed in this study. The additional domain is identified as a DNA helicase, and functional deletion of this domain in vivo leads to PT modification deficiency, indicating an essential role of helicase activity in PT modification. Subsequent analysis reveals that the additional domain has an ATPase activity. Intriguingly, the ATPase activity is strongly stimulated by DNA substrate containing a GAAC/GTTC motif (i.e. the motif at which PT modifications occur in R. anatipestifer) when the additional domain and the other domain (homologous to canonical DndE) are co-expressed as a full-length DndEi. These results reveal that PT modification is a biochemical process with DNA strand separation and intense ATP hydrolysis.

In vitro analysis of phosphorothioate modification of DNA reveals substrate recognition by a multiprotein complex

A wide variety of prokaryotes possess DNA modifications consisting of sequence-specific phosphorothioates (PT) inserted by members of a five-gene cluster. Recent genome mapping studies revealed two unusual features of PT modifications: short consensus sequences and partial modification of a specific genomic site in a population of bacteria. To better understand the mechanism of target selection of PT modifications that underlies these features, we characterized the substrate recognition of the PT-modifying enzymes termed DptC, D and E in a cell extract system from Salmonella. The results revealed that double-stranded oligodeoxynucleotides underwent de novo PT modification in vitro, with the same modification pattern as in vivo, i. e., G_psAAC/G_psTTC motif. Unexpectedly, in these in vitro analyses we observed no significant effect on PT modification by sequences flanking GAAC/GTTC motif, while PT also occurred in the GAAC/GTTC motif that could not be modified in vivo. Hemi-PT DNA also served as substrate of the PT-modifying enzymes, but not single-stranded DNA. The PT-modifying enzymes were then found to function as a large protein complex, with all of three subunits in tetrameric conformations. This study provided the first demonstration of in vitro DNA PT modification by PT-modifying enzymes that function as a large protein complex.

Regulation of DNA phosphorothioate modification in Salmonella enterica by DndB

DNA phosphorothioate (PT) modification, in which the non-bridging oxygen of the sugar-phosphate backbone is substituted by sulfur, occurs naturally in diverse bacteria and archaea and is regulated by the DndABCDE proteins. DndABCDE and the restriction cognate DndFGHI constitute a new type of defense system that prevents the invasion of foreign DNA in Salmonella enterica serovar Cerro 87. GAAC/GTTC consensus contexts across genomes were found to possess partial PT modifications even in the presence of restriction activity, indicating the regulation of PT. The abundance of PT in cells must be controlled to suit cellular activities. However, the regulatory mechanism of PT modification has not been characterized. The result here indicated that genomic PT modification in S. enterica is controlled by the transcriptional regulator DndB, which binds to two regions in the promoter, each possessing a 5'-TACGN(10)CGTA-3' palindromic motif, to regulate the transcription of dndCDE and its own gene. Site-directed mutagenesis showed that the Cys29 residue of DndB plays a key role in its DNA-binding activity or conformation. Proteomic analysis identified changes to a number of cellular proteins upon up-regulation and loss of PT. Considering the genetic conservation of dnd operons, regulation of PT by DndB might be widespread in diverse organisms.

Regulation of DNA phosphorothioate modifications by the transcriptional regulator DptB in Salmonella

DNA phosphorothioate (PT) modifications, with one non-bridging phosphate oxygen replaced with sulfur, are widely but sporadically distributed in prokaryotic genomes. Short consensus sequences surround the modified linkage in each strain, although each site is only partially modified. The mechanism that maintains this low-frequency modification status is still unknown. In Salmonella enterica serovar Cerro 87, PT modification is mediated by a four-gene cluster called dptBCDE. Here, we found that deletion of dptB led to a significant increase in intracellular PT modification level. In this deletion, transcription of downstream genes was elevated during rapid cell growth. Restoration of dptB on a plasmid restored wild-type levels of expression of downstream genes and PT modification. In vitro, DptB directly protected two separate sequences within the dpt promoter region from DNase I cleavage. Each protected sequence contained a direct repeat (DR). Mutagenesis assays of the DRs demonstrated that each DR was essential for DptB binding. The observation of two shifted species by gel-shift analysis suggests dimer conformation of DptB protein. These DRs are conserved among the promoter regions of dptB homologs, suggesting that this regulatory mechanism is widespread. These findings demonstrate that PT modification is regulated at least in part at the transcriptional level.

Crystallization and preliminary X-ray analysis of the type IV restriction endonuclease ScoMcrA from Streptomyces coelicolor, which cleaves both Dcm-methylated DNA and phosphorothioated DNA

ScoMcrA is a type IV modification-dependent restriction endonuclease found in the model strain Streptomyces coelicolor. Unlike type I, II and III restriction endonucleases, which cleave unmodified DNA, type IV restriction endonucleases cleave modified DNA, including methylated, hydroxymethylated, glucosyl-hydroxymethylated and phosphorothioated DNA. ScoMcrA targets both Dcm-methylated DNA and phosphorothioated DNA, and makes double-strand breaks 16-28 nt away from the modified nucleotides or the phosphorothioate links. However, the mechanism by which ScoMcrA recognizes these two entirely different types of modification remains unclear. In this study, the ScoMcrA protein was overexpressed, purified and crystallized. The crystals diffracted to 3.35 Å resolution and belonged to space group P2(1)2(1)2(1). The unit-cell parameters were determined to be a=130.19, b=139.36, c=281.01 Å, α=β=γ=90°. These results will facilitate the detailed structural analysis of ScoMcrA and further elucidation of its biochemical mechanism.