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

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.

T5-like phage BF23 evades host-mediated DNA restriction and methylation

Bacteriophage BF23 is a close relative of phage T5, a prototypical Tequintavirus that infects Escherichia coli. BF23 was isolated in the middle of the XXth century and was extensively studied as a model object. Like T5, BF23 carries long ∼9.7 kb terminal repeats, injects its genome into infected cell in a two-stage process, and carries multiple specific nicks in its double-stranded genomic DNA. The two phages rely on different host secondary receptors-FhuA (T5) and BtuB (BF23). Only short fragments of the BF23 genome, including the region encoding receptor interacting proteins, have been determined. Here, we report the full genomic sequence of BF23 and describe the protein content of its virion. T5-like phages represent a unique group that resist restriction by most nuclease-based host immunity systems. We show that BF23, like other Tequintavirus phages, resist Types I/II/III restriction-modification host immunity systems if their recognition sites are located outside the terminal repeats. We also demonstrate that the BF23 avoids host-mediated methylation. We propose that inhibition of methylation is a common feature of Tequintavirus and Epseptimavirus genera phages, that is not, however, associated with their antirestriction activity.

The highly diverse antiphage defence systems of bacteria

Bacteria and their viruses have coevolved for billions of years. This ancient and still ongoing arms race has led bacteria to develop a vast antiphage arsenal. The development of high-throughput screening methods expanded our knowledge of defence systems from a handful to more than a hundred systems, unveiling many different molecular mechanisms. These findings reveal that bacterial immunity is much more complex than previously thought. In this Review, we explore recently discovered bacterial antiphage defence systems, with a particular focus on their molecular diversity, and discuss the ecological and evolutionary drivers and implications of the existing diversity of antiphage defence mechanisms.

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.

CasDinG is a 5'-3' dsDNA and RNA/DNA helicase with three accessory domains essential for type IV CRISPR immunity

CRISPR-associated DinG protein (CasDinG) is essential to type IV-A CRISPR function. Here, we demonstrate that CasDinG from Pseudomonas aeruginosa strain 83 is an ATP-dependent 5'-3' DNA translocase that unwinds double-stranded (ds)DNA and RNA/DNA hybrids. The crystal structure of CasDinG reveals a superfamily 2 helicase core of two RecA-like domains with three accessory domains (N-terminal, arch, and vestigial FeS). To examine the in vivo function of these domains, we identified the preferred PAM sequence for the type IV-A system (5'-GNAWN-3' on the 5'-side of the target) with a plasmid library and performed plasmid clearance assays with domain deletion mutants. Plasmid clearance assays demonstrated that all three domains are essential for type IV-A immunity. Protein expression and biochemical assays suggested the vFeS domain is needed for protein stability and the arch for helicase activity. However, deletion of the N-terminal domain did not impair ATPase, ssDNA binding, or helicase activities, indicating a role distinct from canonical helicase activities that structure prediction tools suggest involves interaction with dsDNA. This work demonstrates CasDinG helicase activity is essential for type IV-A CRISPR immunity as well as the yet undetermined activity of the CasDinG N-terminal domain.

Recent advances in phage defense systems and potential overcoming strategies

Bacteriophages are effective in the prevention and control of bacteria, and many phage products have been permitted and applied in the field. Because bacteriophages are expected to replace other antimicrobial agents like antibiotics, the antibacterial effect of bacteriophage has attracted widespread attention. Recently, the diversified defense systems discovered in the target host have become potential threats to the continued effective application of phages. Therefore, a systematic summary and in-depth illustration of the interaction between phages and bacteria is conducive to the development of this biological control approach. In this review, we introduce different defense systems in bacteria against phages and emphasize newly discovered defense mechanisms in recent years. Additionally, we draw attention to the striking resemblance between defense system genes and antibiotic resistance genes, which raises concerns about the potential transfer of phage defense systems within bacterial populations and its future impact on phage efficacy. Thus, attention should be given to the effects of phage defense genes in practical applications. This article is not exhaustive, but strategies to overcome phage defense systems are also discussed to further promote more efficient use of phages.

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.

Uncovering the determinants of model Escherichia coli strain C600 susceptibility and resistance to lytic T4-like and T7-like phage

As antimicrobial resistance (AMR) continues to increase, the therapeutic use of phages has re-emerged as an attractive alternative. However, knowledge of phage resistance development and bacterium-phage interaction complexity are still not fully interpreted. In this study, two lytic T4-like and T7-like phage infecting model Escherichia coli strain C600 are selected, and host genetic determinants involved in phage susceptibility and resistance are also identified using TraDIS strategy. Isolation and identification of the lytic T7-like show that though it belongs to the phage T7 family, genes encoding replication and transcription protein exhibit high differences. The TraDIS results identify a huge number of previously unidentified genes involved in phage infection, and a subset (six in susceptibility and nine in resistance) are shared under pressure of the two kinds of lytic phage. Susceptible gene wbbL has the highest value and implies the important role in phage susceptibility. Importantly, two susceptible genes QseE (QseE/QseF) and RstB (RstB/RstA), encoding the similar two-component system sensor histidine kinase (HKs), also identified. Conversely and strangely, outer membrane protein gene ompW, unlike the gene ompC encoding receptor protein of T4 phage, was shown to provide phage resistance. Overall, this study exploited a genome-wide fitness assay to uncover susceptibility and resistant genes, even the shared genes, important for the E. coli strain of both most popular high lytic T4-like and T7-like phages. This knowledge of the genetic determinants can be further used to analysis the behind function signatures to screen the potential agents to aid phage killing of MDR pathogens, which will greatly be valuable in improving the phage therapy outcome in fighting with microbial resistance.

Systematic strategies for developing phage resistant Escherichia coli strains

Phages are regarded as powerful antagonists of bacteria, especially in industrial fermentation processes involving bacteria. While bacteria have developed various defense mechanisms, most of which are effective against a narrow range of phages and consequently exert limited protection from phage infection. Here, we report a strategy for developing phage-resistant Escherichia coli strains through the simultaneous genomic integration of a DNA phosphorothioation-based Ssp defense module and mutations of components essential for the phage life cycle. The engineered E. coli strains show strong resistance against diverse phages tested without affecting cell growth. Additionally, the resultant engineered phage-resistant strains maintain the capabilities of producing example recombinant proteins, D-amino acid oxidase and coronavirus-encoded nonstructural protein nsp8, even under high levels of phage cocktail challenge. The strategy reported here will be useful for developing engineered E. coli strains with improved phage resistance for various industrial fermentation processes for producing recombinant proteins and chemicals of interest.

Phage contamination is a persistent problem in industrial biotechnology processes employing bacterial strains. Here, the authors report the construction of E. coli host strains with broad antiphase activities via the genomic integration of the Ssp defense system and mutations of components essential for phage infection cycles.

PADLOC: a web server for the identification of antiviral defence systems in microbial genomes

Most bacteria and archaea possess multiple antiviral defence systems that protect against infection by phages, archaeal viruses and mobile genetic elements. Our understanding of the diversity of defence systems has increased greatly in the last few years, and many more systems likely await discovery. To identify defence-related genes, we recently developed the Prokaryotic Antiviral Defence LOCator (PADLOC) bioinformatics tool. To increase the accessibility of PADLOC, we describe here the PADLOC web server (freely available at https://padloc.otago.ac.nz), allowing users to analyse whole genomes, metagenomic contigs, plasmids, phages and archaeal viruses. The web server includes a more than 5-fold increase in defence system types detected (since the first release) and expanded functionality enabling detection of CRISPR arrays and retron ncRNAs. Here, we provide user information such as input options, description of the multiple outputs, limitations and considerations for interpretation of the results, and guidance for subsequent analyses. The PADLOC web server also houses a precomputed database of the defence systems in > 230,000 RefSeq genomes. These data reveal two taxa, Campylobacterota and Spriochaetota, with unusual defence system diversity and abundance. Overall, the PADLOC web server provides a convenient and accessible resource for the detection of antiviral defence systems.

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.

How Streptomyces thrive: Advancing our understanding of classical development and uncovering new behaviors

Streptomyces are soil- and marine-dwelling microbes that need to survive dramatic fluctuations in nutrient levels and environmental conditions. Here, we explore the advances made in understanding how Streptomyces bacteria can thrive in their natural environments. We examine their classical developmental cycle, and the intricate regulatory cascades that govern it. We discuss alternative growth strategies and behaviors, like the rapid expansion and colonization properties associated with exploratory growth, the release of membrane vesicles and S-cells from hyphal tips, and the acquisition of exogenous DNA along the lateral walls. We further investigate Streptomyces interactions with other organisms through the release of volatile compounds that impact nutrient levels, microbial growth, and insect behavior. Finally, we explore the increasingly diverse strategies employed by Streptomyces species in escaping and thwarting phage infections.

Mechanisms and clinical importance of bacteriophage resistance

We are in the midst of a golden age of uncovering defense systems against bacteriophages. Apart from the fundamental interest in these defense systems, and revolutionary applications that have been derived from them (e.g. CRISPR-Cas9 and restriction endonucleases), it is unknown how defense systems contribute to resistance formation against bacteriophages in clinical settings. Bacteriophages are now being reconsidered as therapeutic agents against bacterial infections due the emergence of multidrug resistance. However, bacteriophage resistance through defense systems and other means could hinder the development of successful phage-based therapies. Here, we review the current state of the field of bacteriophage defense, highlight the relevance of bacteriophage defense for potential clinical use of bacteriophages as therapeutic agents and suggest new directions of research.