Prokaryotic innate immunity through pattern recognition of conserved viral proteins

Exploring deep learning in phage discovery and characterization

Bacteriophages, or bacterial viruses, play diverse ecological roles by shaping bacterial populations and also hold significant biotechnological and medical potential, including the treatment of infections caused by multidrug-resistant bacteria. The discovery of novel bacteriophages using large-scale metagenomic data has been accelerated by the accessibility of deep learning (Artificial Intelligence), the increased computing power of graphical processing units (GPUs), and new bioinformatics tools. This review addresses the recent revolution in bacteriophage research, ranging from the adoption of neural network algorithms applied to metagenomic data to the use of pre-trained language models, such as BERT, which have improved the reconstruction of viral metagenome-assembled genomes (vMAGs). This article also discusses the main aspects of bacteriophage biology using deep learning, highlighting the advances and limitations of this approach. Finally, prospects of deep-learning-based metagenomic algorithms and recommendations for future investigations are described.

Functional characterization of Mrr-family nuclease SLL1429 involved in MMC and phage resistance

Cyanobacteria, autotrophic prokaryotes capable of oxygenic photosynthesis, are important atmospheric carbon fixers of Earth and potential alternatives for producing green fuels and chemicals. However, they face significant environmental stress during growth, such as Ultraviolet radiation, salt, and cyanophage exposure, which can impact their physiology and growth. Nucleases, such as Mrr (Methylated adenine Recognition and Restriction) endonuclease, play key roles in stress response, DNA repair, or anti-phage functions, but these in cyanobacteria remains underexplored. The SLL1429 protein with Mrr/NA-iREase1 domain was predicted to play a role as a nuclease in stress resistance in cyanobacteria. In this study, our findings indicate that SLL1429 is a PD-(D/E)XK superfamily nuclease with DNase activities towards various DNA structures, including dsDNA, Holliday junction, Flap and Flap derivatives. The nuclease activity of SLL1429 is dependent on the Mrr domain. However, unlike classic Mrr, SLL1429 recognizes and cleaves both methylated and unmethylated DNA substrates. Notably, SLL1429 plays a role in Mitomycin C (MMC) resistance in Synechocystis sp. PCC6803 and anti-phage activity in E. coli. In view of the above, SLL1429 of Synechocystis sp. PCC6803 has been identified as a new stress-resistant nuclease. This discovery provides novel perspectives on the mechanism of environmental adaption in cyanobacteria and lays a theoretical foundation for further exploration of "microbial cell factory".

Ancient convergence with prokaryote defense and recent adaptations to lentiviruses in primates characterize the ancestral immune factors SAMD9s

Human SAMD9 and SAMD9L are duplicated genes that encode innate immune proteins restricting poxviruses and lentiviruses, such as HIV, and implicated in life-threatening genetic diseases and cancer. Here, we combined structural similarity searches, phylogenetics and population genomics with experimental assays of SAMD9/9L functions to resolve the evolutionary and functional dynamics of these immune proteins, spanning from prokaryotes to primates. We discovered structural analogs of SAMD9/9L in the anti-bacteriophage defense system Avs, resulting from convergent evolution. Further, the predicted nuclease active site was conserved in bacterial analogs and was essential for cell death functions, suggesting a fundamental role in defense across different life kingdoms. Despite this ancestral immunity, we identified genomic signatures of evolutionary arms-races in mammals, with remarkable gene copy number variations targeted by natural selection. We further unveiled that the absence of SAMD9 in bonobos corresponds to a recent gene loss still segregating in the population. Finally, we found that chimp and bonobo SAMD9Ls have enhanced anti-HIV-1 functions, and that bonobo-specific SAMD9L polymorphisms confer increased anti-HIV-1 activity to human SAMD9L without compromising its effect on cell translation. These SAMD9/9L adaptations likely resulted from strong viral selective pressures, including by primate lentiviruses, and could contribute to lentiviral resistance in bonobos. Altogether, this study elucidates the interplay between ancient immune convergence across kingdoms and species-specific adaptations within the Avs9 and SAMD9/9L antiviral shared immunity.

Ferroptosis in plant immunity

Plant cell death is mediated by calcium, iron, and reactive oxygen species (ROS) signaling in plant immunity. The reconstruction of a nucleotide-binding leucine-rich-repeat receptor (NLR) supramolecular structure, called the resistosome, is intimately involved in the hypersensitive response (HR), a type of cell death involved in effector-triggered immunity (ETI). Iron is a crucial redox catalyst in various cellular reactions. Ferroptosis is a regulated, non-apoptotic form of iron- and ROS-dependent cell death in plants. Pathogen infections trigger iron accumulation and ROS bursts in plant cells, leading to lipid peroxidation via the Fenton reaction and subsequent ferroptosis in plant cells similar to that in mammalian cells. The small-molecule inducer erastin triggers iron-dependent lipid ROS accumulation and glutathione depletion, leading to HR cell death in plant immunity. Calcium (Ca2+) is another major mediator of plant immunity. Cytoplasmic Ca2+ influx through calcium-permeable channels, the resistosomes, mediates iron- and ROS-dependent ferroptotic cell death under reduced glutathione reductase (GR) expression levels in the ETI response. Acibenzolar-S-methyl (ASM), a plant defense activator, enhances Ca2+ influx, ROS and iron accumulation, and lipid peroxidation to trigger ferroptotic cell death. These breakthroughs suggest a potential role for Ca2+ signaling in ferroptosis and its coordination with iron and ROS signaling in plant immunity. In this review, we highlight the essential roles of calcium, iron, and ROS signaling in ferroptosis during plant immunity and discuss advances in the understanding of how Ca2+-mediated ferroptotic cell death orchestrates effective plant immune responses against invading pathogens.

Activation of bacterial programmed cell death by phage inhibitors of host immunity

Bacterial and archaeal viruses are replete with diverse uncharacterized accessory genes (AGs), which likely interface with host processes. However, large-scale discovery of virus AG functions remains challenging. Here, we developed an integrated computational and experimental discovery platform to identify viral AGs and assign functions. We show that multiple AGs activate unexpected programmed cell death (PCD) activity of distinct restriction-modification (R-M) systems. We describe an exapted type I R-M decoy that kills the host upon sensing several different anti-defense AGs and a self-guarded type III R-M system that restricts phages but also induces PCD when bound by anti-R-M proteins. Other phage counter-defense genes additionally activate non-R-M-based abortive infection systems encoded by prophages. This defense strategy creates a conundrum: lose AGs and be exposed to immunity or keep AGs and trigger PCD. Strategies employed by viruses to avoid this double bind could be an important factor in virus evolution that remains to be explored.

DnaJ mediates phage sensing by the bacterial NLR-related protein bNACHT25

Bacteria encode a wide range of antiphage systems and a subset of these proteins are homologous to components of the human innate immune system. Mammalian nucleotide-binding and leucine-rich repeat containing proteins (NLRs) and bacterial NLR-related proteins use a central NACHT domain to link detection of infection with initiation of an antimicrobial response. Bacterial NACHT proteins provide defense against both DNA and RNA phages. Here we investigate the mechanism of phage detection by the bacterial NLR-related protein bNACHT25 in E. coli. bNACHT25 was specifically activated by Emesvirus ssRNA phages and analysis of MS2 phage escaper mutants that evaded detection revealed a critical role for Coat Protein (CP). A genetic assay showed CP was sufficient to activate bNACHT25 but the two proteins did not directly interact. Instead, we found bNACHT25 requires the host chaperone DnaJ to detect CP and protect against phage. Our data support a model in which bNACHT25 detects a wide range of phages using an indirect mechanism that may involve guarding a host cell process rather than binding a specific phage-derived molecule.

Advancing RNA phage biology through meta-omics

Bacteriophages with RNA genomes are among the simplest biological entities on Earth. Since their discovery in the 1960s, they have been used as important models to understand the principal processes of life, including translation and the genetic code. While RNA phages were generally thought of as rare oddities in nature, meta-omics methods are rapidly changing this simplistic view by studying diverse biomes with unprecedented resolution. Metatranscriptomics dramatically expanded the number of known RNA phages from tens to tens of thousands, revealed their widespread abundance, and discovered several new families of potential RNA phages with largely unknown hosts, biology, and environmental impact. At the same time, (meta)genomic analyses of bacterial hosts are discovering an arsenal of defense systems bacteria employ to protect themselves from predation, whose functions in immunity against RNA phages we are only beginning to understand. Here, I review how meta-omics approaches are advancing the field of RNA phage biology with a focus on the discovery of new RNA phages and how bacteria might fight them.

Reprogrammable RNA-targeting CRISPR systems evolved from RNA toxin-antitoxins

Despite ongoing efforts to study CRISPR systems, the evolutionary origins giving rise to reprogrammable RNA-guided mechanisms remain poorly understood. Here, we describe an integrated sequence/structure evolutionary tracing approach to identify the ancestors of the RNA-targeting CRISPR-Cas13 system. We find that Cas13 likely evolved from AbiF, which is encoded by an abortive infection-linked gene that is stably associated with a conserved non-coding RNA (ncRNA). We further characterize a miniature Cas13, classified here as Cas13e, which serves as an evolutionary intermediate between AbiF and other known Cas13s. Despite this relationship, we show that their functions substantially differ. Whereas Cas13e is an RNA-guided RNA-targeting system, AbiF is a toxin-antitoxin (TA) system with an RNA antitoxin. We solve the structure of AbiF using cryoelectron microscopy (cryo-EM), revealing basic structural alterations that set Cas13s apart from AbiF. Finally, we map the key structural changes that enabled a non-guided TA system to evolve into an RNA-guided CRISPR system.

Structural basis for Lamassu-based antiviral immunity and its evolution from DNA repair machinery

Bacterial immune systems exhibit remarkable diversity and modularity, as a consequence of the continuous selective pressures imposed by phage predation. Despite recent mechanistic advances, the evolutionary origins of many antiphage immune systems remain elusive, especially for those that encode homologs of the Structural Maintenance of Chromosomes (SMC) superfamily, which are essential for chromosome maintenance and DNA repair across domains of life. Here, we elucidate the structural basis and evolutionary emergence of Lamassu, a bacterial immune system family featuring diverse effectors but a core conserved SMC-like sensor. Using cryo-EM, we determined structures of the Vibrio cholerae Lamassu complex in both apo- and dsDNA-bound states, revealing unexpected stoichiometry and topological architectures. We further demonstrate how Lamassu specifically senses dsDNA in vitro and phage replication origins in vivo , thereby triggering the formation of LmuA tetramers that activate the Cap4 nuclease domain. Our findings reveal that Lamassu evolved via exaptation of the bacterial Rad50-Mre11 DNA repair system to form a compact, modular sensor for viral replication, exemplifying how cellular machinery can be co-opted for novel immune functions.

Architecture remodeling activates the HerA-DUF anti-phage defense system

Leveraging AlphaFold models and integrated experiments, we characterized the HerA-DUF4297 (DUF) anti-phage defense system, focusing on DUF's undefined biochemical functions. Guided by structure-based genomic analyses, we found DUF homologs to be universally distributed across diverse bacterial immune systems. Notably, one such homolog, Cap4, is a nuclease. Inspired by this evolutionary clue, we tested DUF's nuclease activity and observed that DUF cleaves DNA substrates only when bound to its partner protein HerA. To dissect the mechanism of DUF activation, we determined the structures of DUF and HerA-DUF. Although DUF forms large oligomeric assemblies both alone and with HerA, oligomerization alone was insufficient to elicit nuclease activity. Instead, HerA binding induces a profound architecture remodeling that propagates throughout the complex. This remodeling reconfigures DUF into an active nuclease capable of robust DNA cleavage. Together, we highlight an architecture remodeling-driven mechanism that may inform the activation of other immune systems.

DUF4297 and HerA form abortosome to mediate bacterial immunity against phage infection

Immune receptors form higher-order complexes known as inflammasomes in animals and resistosomes in plants to mediate immune signaling. Here, we report a similar bacterial protein complex, DUF4297-HerA, which induces abortive infection to mediate anti-phage immunity by coupling nuclease and ATPase activities. Therefore, we name this defense system "Hailibu" after a hunter in a popular folk tale who sacrifices himself to save his village. Cryoelectron microscopy (cryo-EM) results reveal that DUF4297 and HerA assemble into a higher-order complex, reminiscent of apoptosome, inflammasome, or resistosome, which we refer to as an abortosome. By capturing cryo-EM structures of the pre-loading, DNA-loading, and DNA-transporting states during Hailibu abortosome processing of DNA, we propose that DNA substrates are loaded through the HerA hexamer, with adenosine triphosphate (ATP) hydrolysis providing the energy to transport DNA substrates to the clustered DUF4297 Cap4 nuclease domains for degradation. This study demonstrates the existence of analogous multiprotein complexes in innate immunity across the kingdoms of life.

Bacterial Hachiman complex executes DNA cleavage for antiphage defense

Bacteria have developed a variety of immune systems to combat phage infections. The Hachiman system is a novel prokaryotic antiphage defense system comprising HamA and HamB proteins, which contains the DUF1837 and helicase domains, respectively. However, the defense mechanism remains only partially understood. Here, we present the cryo-electron microscopy (cryo-EM) structure of the Hachiman defense system featuring a fusion of Cap4 nuclease domain within HamA. Further structure analysis indicates that the DUF1837 domain on HamA resembles the PD-(D/E)XK nuclease but lacks active sites. Bioinformatics analysis reveals that catalytically inactive DUF1837 domains often recruit other functional domains to fulfill anti-phage defense. HamA interacts with HamB to form a heterodimer HamAB to mediate ATP hydrolysis and execute DNA cleavage, thus implementing antiphage defense. Our findings elucidate the structural basis of the Hachiman defense complex, highlighting the critical roles of the helicase and nuclease in prokaryotic immunity.

Filamentation activates bacterial Avs5 antiviral protein

Bacterial antiviral STANDs (Avs) are evolutionarily related to the nucleotide-binding oligomerization domain (NOD)-like receptors widely distributed in immune systems across animals and plants. EfAvs5, a type 5 Avs from Escherichia fergusonii, contains an N-terminal SIR2 effector domain, a NOD, and a C-terminal sensor domain, conferring protection against diverse phage invasions. Despite the established roles of SIR2 and STAND in prokaryotic and eukaryotic immunity, the mechanism underlying their collaboration remains unclear. Here we present cryo-EM structures of EfAvs5 filaments, elucidating the mechanisms of dimerization, filamentation, filament bundling, ATP binding, and NAD+ hydrolysis, all of which are crucial for anti-phage defense. The SIR2 and NOD domains engage in intra- and inter-dimer interaction to form an individual filament, while the outward C-terminal sensor domains contribute to bundle formation. Filamentation potentially stabilizes the dimeric SIR2 configuration, thereby activating the NADase activity of EfAvs5. Furthermore, we identify the nucleotide kinase gp1.7 of phage T7 as an activator of EfAvs5, demonstrating its ability to induce filamentation and NADase activity. Together, we uncover the filament assembly of Avs5 as a unique mechanism to switch enzyme activities and perform anti-phage defenses.

ChNLRC4, a cytoplasmic pattern recognition receptor, activates the pyroptosis signaling pathway in Mollusca

NLR inflammasomes recognize pathogen-associated molecular patterns (PAMPs), triggering Caspase-1 activation and leading to gasdermin D (GSDMD)-mediated pyroptosis, a crucial immune response in mammals. The functional GSDME-mediated pyroptosis has been reported in invertebrates, yet the existence of an NLR-Caspase-GSDME axis mediating pyroptosis signaling cascades remains unclear. In this study, we reported an NLRC4 homolog named ChNLRC4, a pattern recognition receptor from the oyster Crassostrea hongkongensis that is able to bind to LPS and Lys-type PGN through its LRR domain. ChNLRC4 interacted with ChCaspase-1 through CARD-CARD domain homotypic interactions and enhanced ChCaspase-1 activity. Additionally, overexpression of ChNLRC4 promoted ChCaspase-1-mediated cleavage of ChGSDME, leading to pyroptosis in HEK293T cells. Furthermore, knockdown of chnlrc4 resulted in a significant reduction in the death rate of hemocytes, immune infiltration of hemocytes, cilium shedding, and bacterial clearance. Collectively, this study provides insight into the role of NLR within the pyroptosis signaling pathway in oysters.

CARD domains mediate anti-phage defence in bacterial gasdermin systems

Caspase recruitment domains (CARDs) and pyrin domains are important facilitators of inflammasome activity and pyroptosis1. Following pathogen recognition by nucleotide binding-domain, leucine-rich, repeat-containing (NLR) proteins, CARDs recruit and activate caspases, which, in turn, activate gasdermin pore-forming proteins to induce pyroptotic cell death2. Here we show that CARD domains are present in defence systems that protect bacteria against phage. The bacterial CARD domain is essential for protease-mediated activation of certain bacterial gasdermins, which promote cell death once phage infection is recognized. We further show that multiple anti-phage defence systems use CARD domains to activate a variety of cell death effectors, and that CARD domains mediate protein-protein interactions in these systems. We find that these systems are triggered by a conserved immune-evasion protein used by phages to overcome the bacterial defence system RexAB3, demonstrating that phage proteins inhibiting one defence system can activate another. Our results suggest that CARD domains represent an ancient component of innate immune systems conserved from bacteria to humans, and that CARD-dependent activation of gasdermins is shared in organisms across the tree of life.

Activation of the bacterial defense-associated sirtuin system

The NADase activity of the defense-associated sirtuins (DSRs) is activated by the phage tail tube protein (TTP). Herein, we report cryo-EM structures of a free-state Bacillus subtilis DSR2 tetramer and a fragment of the tetramer, a phage SPR tail tube, and two DSR2-TTP complexes. DSR2 contains an N-terminal SIR2 domain, a middle domain (MID) and a C-terminal domain (CTD). The DSR2 CTD harbors the α-solenoid tandem-repeats like the HEAT-repeat proteins. DSR2 assembles into a tetramer with four SIR2 clustered at the center, and two intertwined MID-CTD chains flank the SIR2 core. SPR TTPs self-assemble into a tube-like complex. Upon DSR2 binding, the D1 domain of SPR TTP is captured between the HEAT-repeats domains of DSR2, which conflicts with TTPs self-assembly. Binding of TTPs induces conformational changes in DSR2 tetramer, resulting in increase of the NAD+ pocket volume in SIR2, thus activates the NADase activity and leads to cellular NAD+ depletion.

Phage engineering to overcome bacterial Tmn immunity in Dhillonvirus

Bacteria possess numerous defense systems against phage infections, which limit phage infectivity and pose challenges for phage therapy. This study aimed to engineer phages capable of evading these defense systems, using the Tmn defense system as a model. We identified an anti-Tmn protein in the ΦSMS22 phage from the Dhillonvirus genus that inhibits Tmn function in Escherichia coli. Introducing this gene into the Tmn-sensitive ΦKSS9 phage enabled it to evade Tmn immunity. Additionally, we found that a single mutation in the nmad5 gene, a DNA modification enzyme in Dhillonvirus, prevented Tmn from sensing phage infection. By mutating the nmad5 gene in the Tmn-sensitive Dhillonvirus, we demonstrated that engineering phages to evade bacterial sensing mechanisms is another viable strategy. These two phage engineering approaches-introducing anti-defense genes and mutating sensing-related genes-present a promising strategy for establishing effective phage therapy by neutralizing bacterial defense systems.

DNA end sensing and cleavage by the Shedu anti-phage defense system

The detection of molecular patterns associated with invading pathogens is a hallmark of innate immune systems. Prokaryotes deploy sophisticated host defense mechanisms in innate anti-phage immunity. Shedu is a single-component defense system comprising a putative nuclease SduA. Here, we report cryoelectron microscopy (cryo-EM) structures of apo- and double-stranded DNA (dsDNA)-bound tetrameric SduA assemblies, revealing that the N-terminal domains of SduA form a clamp that recognizes free DNA ends. End binding positions the DNA over the PD-(D/E)XK nuclease domain, resulting in dsDNA nicking at a fixed distance from the 5' end. The end-directed DNA nicking activity of Shedu prevents propagation of linear DNA in vivo. Finally, we show that phages escape Shedu immunity by suppressing their recombination-dependent DNA replication pathway. Taken together, these results define the antiviral mechanism of Shedu systems, underlining the paradigm that recognition of pathogen-specific nucleic acid structures is a conserved feature of innate immunity across all domains of life.

All Roads Lead to Rome: Pathways to Engineering Disease Resistance in Plants

Unlike animals, plants are unable to move and lack specialized immune cells and circulating antibodies. As a result, they are always threatened by a large number of microbial pathogens and harmful pests that can significantly reduce crop yield worldwide. Therefore, the development of new strategies to control them is essential to mitigate the increasing risk of crops lost to plant diseases. Recent developments in genetic engineering, including efficient gene manipulation and transformation methods, gene editing and synthetic biology, coupled with the understanding of microbial pathogenicity and plant immunity, both at molecular and genomic levels, have enhanced the capabilities to develop disease resistance in plants. This review comprehensively explains the fundamental mechanisms underlying the tug-of-war between pathogens and hosts, and provides a detailed overview of different strategies for developing disease resistance in plants. Additionally, it provides a summary of the potential genes that can be employed in resistance breeding for key crops to combat a wide range of potential pathogens and pests, including fungi, oomycetes, bacteria, viruses, nematodes, and insects. Furthermore, this review addresses the limitations associated with these strategies and their possible solutions. Finally, it discusses the future perspectives for producing plants with durable and broad-spectrum disease resistance.

TIR signaling activates caspase-like immunity in bacteria

Caspase family proteases and Toll/interleukin-1 receptor (TIR)-domain proteins have central roles in innate immunity and regulated cell death in humans. We describe a bacterial immune system comprising both a caspase-like protease and a TIR-domain protein. We found that the TIR protein, once it recognizes phage invasion, produces the previously unknown immune signaling molecule adenosine 5'-diphosphate-cyclo[N7:1'']-ribose (N7-cADPR). This molecule specifically activates the bacterial caspase-like protease, which then indiscriminately degrades cellular proteins to halt phage replication. The TIR-caspase defense system, which we denote as type IV Thoeris, is abundant in bacteria and efficiently protects against phage propagation. Our study highlights the diversity of TIR-produced immune signaling molecules and demonstrates that cell death regulated by proteases of the caspase family is an ancient mechanism of innate immunity.

Nucleic acid recognition during prokaryotic immunity

Parasitic elements often spread to hosts through the delivery of their nucleic acids to the recipient. This is particularly true for the primary parasites of bacteria, bacteriophages (phages) and plasmids. Although bacterial immune systems can sense a diverse set of infection signals, such as a protein unique to the invader or the disruption of natural host processes, phage and plasmid nucleic acids represent some of the most common molecules that are recognized as foreign to initiate defense. In this review, we will discuss the various elements of invader nucleic acids that can be distinguished by bacterial host immune systems as "non-self" and how this signal is relayed to activate an immune response.

Reconstructing NOD-like receptor alleles with high internal conservation in Podospora anserina using long-read sequencing

NOD-like receptors (NLRs) are intracellular immune receptors that detect pathogen-associated cues and trigger defense mechanisms, including regulated cell death. In filamentous fungi, some NLRs mediate heterokaryon incompatibility, a self/non-self recognition process that prevents the vegetative fusion of genetically distinct individuals, reducing the risk of parasitism. The het-d and het-e NLRs in Podospora anserina are highly polymorphic incompatibility genes (het genes) whose products recognize different alleles of the het-c gene via a sensor domain composed of WD40 repeats. These repeats display unusually high sequence identity maintained by concerted evolution. However, some sites within individual repeats are hypervariable and under diversifying selection. Despite extensive genetic studies, inconsistencies in the reported WD40 domain sequence have hindered functional and evolutionary analyses. Here we demonstrate that the WD40 domain can be accurately reconstructed from long-read sequencing (Oxford Nanopore and PacBio) data, but not from Illumina-based assemblies. Functional alleles are usually formed by 11 highly conserved repeats, with different repeat combinations underlying the same phenotypic het-d and het-e incompatibility reactions. Protein structure models suggest that their WD40 domain folds into two 7-blade β-propellers composed of the highly conserved repeats, as well as three cryptic divergent repeats at the C-terminus. We additionally show that one particular het-e allele does not have an incompatibility reaction with common het-c alleles, despite being 11-repeats long. Our findings provide a robust foundation for future research into the molecular mechanisms and evolutionary dynamics of het NLRs, while also highlighting both the fragility and the flexibility of β-propellers as immune sensor domains.

Exploring the diversity of anti-defense systems across prokaryotes, phages and mobile genetic elements

The co-evolution of prokaryotes, phages and mobile genetic elements (MGEs) has driven the diversification of defense and anti-defense systems alike. Anti-defense proteins have diverse functional domains, sequences and are typically small, creating a challenge to detect anti-defense homologs across prokaryotic and phage genomes. To date, no tools comprehensively annotate anti-defense proteins within a desired sequence. Here, we developed 'AntiDefenseFinder'-a free open-source tool and web service that detects 156 anti-defense systems of one or more proteins in any genomic sequence. Using this dataset, we identified 47 981 anti-defense systems distributed across prokaryotes and their viruses. We found that some genes co-localize in 'anti-defense islands', including Escherichia coli T4 and Lambda phages, although many appear standalone. Eighty-nine per cent anti-defense systems localize only or preferentially in MGE. However, >80% of anti-Pycsar protein 1 (Apyc1) resides in nonmobile regions of bacterial genomes. Evolutionary analysis and biochemical experiments revealed that Apyc1 likely originated in bacteria to regulate cyclic nucleotide (cNMP) signaling, but phage co-opted Apyc1 to overcome cNMP-utilizing defenses. With the AntiDefenseFinder tool, we hope to facilitate the identification of the full repertoire of anti-defense systems in MGEs, the discovery of new protein functions and a deeper understanding of host-pathogen arms race.

Analysis of regulatory patterns of NLRP3 corpuscles and related genes and the role of macrophage polarization in atherosclerosis based on online database

Our study examined the relationships and interactions among 30 genes related to the NOD-like receptor protein 3 (NLRP3) inflammasome. We identified 368 interconnections between these 30 genes, with NLRP3 participating in 38 interactions. The potential roles of these genes in atherosclerosis were evaluated based on protein-protein interaction networks and coexpression analysis. We identified differential expression in 20 genes, five of which were significantly upregulated: P2RX7, CASP1, CD36, GBP5, and PYCARD. We also observed a strong positive association between P2RX7 and PYCARD and as a notable negative association between RELA and CD36. Furthermore, our analysis revealed a clear association between the expression of inflammasome-associated genes and immune cell infiltration in disease specimens. To diagnose AS, a logistic regression model based on six inflammasome-related genes, achieved an Area under the curve of 0.996, indicating excellent diagnostic performance. Genomic enrichment analysis indicated that inflammasome-related genes were primarily involved in various pathways, such as hypertrophic cardiomyopathy and ribosomal function. To validate our findings, we confirmed the expression of risk genes in AS cells using qRT-PCR and Western blot techniques. Additionally, we observed a shift toward M2 polarization in THP-1 macrophages upon P2RX7 knockdown, further supporting our findings.

Phage detection by a bacterial NLR-related protein is mediated by DnaJ

Bacteria encode a wide range of antiphage systems and a subset of these proteins are homologous to components of the human innate immune system. Mammalian nucleotide-binding and leucine-rich repeat containing proteins (NLRs) and bacterial NLR-related proteins use a central NACHT domain to link detection of infection with initiation of an antimicrobial response. Bacterial NACHT proteins provide defense against both DNA and RNA phages. Here we determine the mechanism of RNA phage detection by the bacterial NLR-related protein bNACHT25 in E. coli. bNACHT25 was specifically activated by Emesvirus ssRNA phages and analysis of MS2 phage escaper mutants that evaded detection revealed a critical role for Coat Protein (CP). A genetic assay confirmed CP was sufficient to activate bNACHT25 but the two proteins did not directly interact. Instead, we found bNACHT25 requires the host chaperone DnaJ to detect CP. Our data suggest that bNACHT25 detects a wide range of phages by guarding a host cell process rather than binding a specific phage-derived molecule.

A bacterial NLR-related protein recognizes multiple unrelated phage triggers to sense infection

Immune systems must rapidly sense viral infections to initiate antiviral signaling and protect the host. Bacteria encode >100 distinct viral (phage) defense systems and each has evolved to sense crucial components or activities associated with the viral lifecycle. Here we used a high-throughput AlphaFold-multimer screen to discover that a bacterial NLR-related protein directly senses multiple phage proteins, thereby limiting immune evasion. Phages encoded as many as 5 unrelated activators that were predicted to bind the same interface of a C-terminal sensor domain. Genetic and biochemical assays confirmed activators bound to the bacterial NLR-related protein at high affinity, induced oligomerization, and initiated signaling. This work highlights how in silico strategies can identify complex protein interaction networks that regulate immune signaling across the tree of life.

NMR resonance assignment of the cell death execution domain BELL2 from multicellular bacterial signalosomes

Signalosomes are high-order protein machineries involved in complex mechanisms controlling regulated immune defense and cell death execution. The immune response is initiated by the recognition of exogeneous or endogenous signals, triggering the signalosome assembly process. The final step of signalosome fate often involves membrane-targeting and activation of pore-forming execution domains, leading to membrane disruption and ultimately cell death. Such cell death-inducing domains have been thoroughly characterized in plants, mammals and fungi, notably for the fungal cell death execution protein domain HeLo. However, little is known on the mechanisms of signalosome-based immune response in bacteria, and the conformation of cell death executors in bacterial signalosomes is still poorly characterized. We recently uncovered the existence of NLR signalosomes in various multicellular bacteria and used genome mining approaches to identify putative cell death executors in Streptomyces olivochromogenes. These proteins contain a C-terminal amyloid domain involved in signal transmission and a N-terminal domain, termed BELL for Bacteria analogous to fungal HeLL (HeLo-like), presumably responsible for membrane-targeting, pore-forming and cell death execution. In the present study, we report the high yield expression of S. olivochromogenes BELL2 and its characterization by solution NMR spectroscopy. BELL is folded in solution and we report backbone and sidechain assignments. We identified five α-helical secondary structure elements and a folded core much smaller than its fungal homolog HeLo. This study constitutes the first step toward the NMR investigation of the full-length protein assembly and its membrane targeting.

Anti-viral defence by an mRNA ADP-ribosyltransferase that blocks translation

Host-pathogen conflicts are crucibles of molecular innovation1,2. Selection for immunity to pathogens has driven the evolution of sophisticated immunity mechanisms throughout biology, including in bacterial defence against bacteriophages3. Here we characterize the widely distributed anti-phage defence system CmdTAC, which provides robust defence against infection by the T-even family of phages4. Our results support a model in which CmdC detects infection by sensing viral capsid proteins, ultimately leading to the activation of a toxic ADP-ribosyltransferase effector protein, CmdT. We show that newly synthesized capsid protein triggers dissociation of the chaperone CmdC from the CmdTAC complex, leading to destabilization and degradation of the antitoxin CmdA, with consequent liberation of the CmdT ADP-ribosyltransferase. Notably, CmdT does not target a protein, DNA or structured RNA, the known targets of other ADP-ribosyltransferases. Instead, CmdT modifies the N6 position of adenine in GA dinucleotides within single-stranded RNAs, leading to arrest of mRNA translation and inhibition of viral replication. Our work reveals a novel mechanism of anti-viral defence and a previously unknown but broadly distributed class of ADP-ribosyltransferases that target mRNA.

The growing repertoire of phage anti-defence systems

The biological interplay between phages and bacteria has driven the evolution of phage anti-defence systems (ADSs), which evade bacterial defence mechanisms. These ADSs bind and inhibit host defence proteins, add covalent modifications and deactivate defence proteins, degrade or sequester signalling molecules utilised by host defence systems, synthesise and restore essential molecules depleted by bacterial defences, or add covalent modifications to phage molecules to avoid recognition. Overall, 145 phage ADSs have been characterised to date. These ADSs counteract 27 of the 152 different bacterial defence families, and we hypothesise that many more ADSs are yet to be discovered. We discuss high-throughput approaches (computational and experimental) which are indispensable for discovering new ADSs and the limitations of these approaches. A comprehensive characterisation of phage ADSs is critical for understanding phage-host interplay and developing clinical applications, such as treatment for multidrug-resistant bacterial infections.

Isolation, Characterization, and Unlocking the Potential of Mimir124 Phage for Personalized Treatment of Difficult, Multidrug-Resistant Uropathogenic E. coli Strain

Escherichia coli and its bacteriophages are among the most studied model microorganisms. Bacteriophages for various E. coli strains can typically be easily isolated from environmental sources, and many of these viruses can be harnessed to combat E. coli infections in humans and animals. However, some relatively rare E. coli strains pose significant challenges in finding suitable phages. The uropathogenic strain E. coli UPEC124, isolated from a patient suffering from neurogenic bladder dysfunction, was found to be resistant to all coliphages in our collections, and initial attempts to isolate new phages failed. Using an improved procedure for phage enrichment, we isolated the N4-related phage Mimir124, belonging to the Gamaleyavirus genus, which was able to lyse this "difficult" E. coli strain. Although Mimir124 is a narrow-spectrum phage, it was effective in the individualized treatment of the patient, leading to pathogen eradication. The primary receptor of Mimir124 was the O antigen of the O101 type; consequently, Mimir124-resistant clones were rough (having lost the O antigen). These clones, however, gained sensitivity to some phages that recognize outer membrane proteins as receptors. Despite the presence of nine potential antiviral systems in the genome of the UPEC124 strain, the difficulty in finding effective phages was largely due to the efficient, non-specific cell surface protection provided by the O antigen. These results highlight the importance of an individualized approach to phage therapy, where narrow host-range phages-typically avoided in pre-fabricated phage cocktails-may be instrumental. Furthermore, this study illustrates how integrating genomic, structural, and functional insights can guide the development of innovative therapeutic strategies, paving the way for broader applications of phage therapy in combating multidrug-resistant bacterial pathogens.

Shared signals, different fates: Calcium and ROS in plant PRR and NLR immunity

Lacking an adaptive immune system, plants rely on innate immunity comprising two main layers: PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI), both utilizing Ca2+ influx and reactive oxygen species (ROS) for signaling. PTI, mediated by pattern-recognition receptors (PRRs), responds to conserved pathogen- or damage-associated molecular patterns. Some pathogens evade PTI using effectors, triggering plants to activate ETI. At the heart of ETI are nucleotide-binding leucine-rich repeat receptors (NLRs), which detect specific pathogen effectors and initiate a robust immune response. NLRs, equipped with a nucleotide-binding domain and leucine-rich repeats, drive a potent immune reaction starting with pronounced, prolonged cytosolic Ca2+ influx, followed by increased ROS levels. This sequence of events triggers the hypersensitive response-a localized cell death designed to limit pathogen spread. This intricate use of Ca2+ and ROS highlights the crucial role of NLRs in supplementing the absence of an adaptive immune system in plant innate immunity.

Diversification of molecular pattern recognition in bacterial NLR-like proteins

Antiviral STANDs (Avs) are bacterial anti-phage proteins evolutionarily related to immune pattern recognition receptors of the NLR family. Type 2 Avs proteins (Avs2) were suggested to recognize the phage large terminase subunit as a signature of phage infection. Here, we show that Avs2 from Klebsiella pneumoniae (KpAvs2) can recognize several different phage proteins as signature for infection. While KpAvs2 recognizes the large terminase subunit of Seuratvirus phages, we find that to protect against Dhillonvirus phages, KpAvs2 recognizes a different phage protein named KpAvs2-stimulating protein 1 (Ksap1). KpAvs2 directly binds Ksap1 to become activated, and phages mutated in Ksap1 escape KpAvs2 defense despite encoding an intact terminase. We further show that KpAvs2 protects against a third group of phages by recognizing another protein, Ksap2. Our results exemplify the evolutionary diversification of molecular pattern recognition in bacterial Avs2, and show that a single pattern recognition receptor evolved to recognize different phage-encoded proteins.

Nucleotide Immune Signaling in CBASS, Pycsar, Thoeris, and CRISPR Antiphage Defense

Bacteria encode an arsenal of diverse systems that defend against phage infection. A common theme uniting many prevalent antiphage defense systems is the use of specialized nucleotide signals that function as second messengers to activate downstream effector proteins and inhibit viral propagation. In this article, we review the molecular mechanisms controlling nucleotide immune signaling in four major families of antiphage defense systems: CBASS, Pycsar, Thoeris, and type III CRISPR immunity. Analyses of the individual steps connecting phage detection, nucleotide signal synthesis, and downstream effector function reveal shared core principles of signaling and uncover system-specific strategies used to augment immune defense. We compare recently discovered mechanisms used by phages to evade nucleotide immune signaling and highlight convergent strategies that shape host-virus interactions. Finally, we explain how the evolutionary connection between bacterial antiphage defense and eukaryotic antiviral immunity defines fundamental rules that govern nucleotide-based immunity across all kingdoms of life.

The Prokaryotic Roots of Eukaryotic Immune Systems

Over the past two decades, studies have revealed profound evolutionary connections between prokaryotic and eukaryotic immune systems, challenging the notion of their unrelatedness. Immune systems across the tree of life share an operational framework, shaping their biochemical logic and evolutionary trajectories. The diversification of immune genes in the prokaryotic superkingdoms, followed by lateral transfer to eukaryotes, was central to the emergence of innate immunity in the latter. These include protein domains related to nucleotide second messenger-dependent systems, NAD+/nucleotide degradation, and P-loop NTPase domains of the STAND and GTPase clades playing pivotal roles in eukaryotic immunity and inflammation. Moreover, several domains orchestrating programmed cell death, ultimately of prokaryotic provenance, suggest an intimate link between immunity and the emergence of multicellularity in eukaryotes such as animals. While eukaryotes directly adopted some proteins from bacterial immune systems, they repurposed others for new immune functions from bacterial interorganismal conflict systems. These emerging immune components hold substantial biotechnological potential.

A bacterial immunity protein directly senses two disparate phage proteins

Eukaryotic innate immune systems use pattern recognition receptors to sense infection by detecting pathogen-associated molecular patterns, which then triggers an immune response. Bacteria have similarly evolved immunity proteins that sense certain components of their viral predators, known as bacteriophages1-6. Although different immunity proteins can recognize different phage-encoded triggers, individual bacterial immunity proteins have been found to sense only a single trigger during infection, suggesting a one-to-one relationship between bacterial pattern recognition receptors and their ligands7-11. Here we demonstrate that the antiphage defence protein CapRelSJ46 in Escherichia coli can directly bind and sense two completely unrelated and structurally different proteins using the same sensory domain, with overlapping but distinct interfaces. Our results highlight the notable versatility of an immune sensory domain, which may be a common property of antiphage defence systems that enables them to keep pace with their rapidly evolving viral predators. We found that Bas11 phages harbour both trigger proteins that are sensed by CapRelSJ46 during infection, and we demonstrate that such phages can fully evade CapRelSJ46 defence only when both triggers are mutated. Our work shows how a bacterial immune system that senses more than one trigger can help prevent phages from easily escaping detection, and it may allow the detection of a broader range of phages. More generally, our findings illustrate unexpected multifactorial sensing by bacterial defence systems and complex coevolutionary relationships between them and their phage-encoded triggers.

Topological rearrangements activate the HerA-DUF anti-phage defense system

Leveraging the rich structural information provided by AlphaFold, we used integrated experimental approaches to characterize the HerA-DUF4297 (DUF) anti-phage defense system, in which DUF is of unknown function. To infer the function of DUF, we performed structure-guided genomic analysis and found that DUF homologs are universally present in bacterial immune defense systems. One notable homolog of DUF is Cap4, a universal effector with nuclease activity in CBASS, the most prevalent anti-phage system in bacteria. To test the inferred nuclease function of DUF, we performed biochemical experiments and discovered that the DUF only exhibits activity against DNA substrates when it is bound by HerA. To understand how HerA activates DUF, we determined the structures of DUF and the HerA-DUF complex. DUF forms large oligomeric assemblies with or without HerA, suggesting that oligomerization per se is not sufficient for DUF activation. Instead, DUF activation requires dramatic topological rearrangements that propagate from HerA to the entire HerA-DUF complex, leading to reorganization of DUF for effective DNA cleavage. We further validated these structural insights by structure- guided mutagenesis. Together, these findings reveal dramatic topological rearrangements throughout the HerA-DUF complex, challenge the long-standing dogma that protein oligomerization alone activates immune signaling, and may inform the activation mechanism of CBASS.

DNA methylation activates retron Ec86 filaments for antiphage defense

Retrons are a class of multigene antiphage defense systems typically consisting of a retron reverse transcriptase, a non-coding RNA, and a cognate effector. Although triggers for several retron systems have been discovered recently, the complete mechanism by which these systems detect invading phages and mediate defense remains unclear. Here, we focus on the retron Ec86 defense system, elucidating its modes of activation and mechanisms of action. We identified a phage-encoded DNA cytosine methyltransferase (Dcm) as a trigger of the Ec86 system and demonstrated that Ec86 is activated upon multicopy single-stranded DNA (msDNA) methylation. We further elucidated the structure of a tripartite retron Ec86-effector filament assembly that is primed for activation by Dcm and capable of hydrolyzing nicotinamide adenine dinucleotide (NAD+). These findings provide insights into the retron Ec86 defense mechanism and underscore an emerging theme of antiphage defense through supramolecular complex assemblies.

Sporadic phage defense in epidemic Vibrio cholerae mediated by the toxin-antitoxin system DarTG is countered by a phage-encoded antitoxin mimic

Bacteria and their viral predators (phages) are constantly evolving to subvert one another. Many bacterial immune systems that inhibit phages are encoded on mobile genetic elements that can be horizontally transmitted to diverse bacteria. Despite the pervasive appearance of immune systems in bacteria, it is not often known if these immune systems function against phages that the host encounters in nature. Additionally, there are limited examples demonstrating how these phages counter-adapt to such immune systems. Here, we identify clinical isolates of the global pathogen Vibrio cholerae harboring a novel genetic element encoding the bacterial immune system DarTG and reveal the immune system's impact on the co-circulating lytic phage ICP1. We show that DarTG inhibits ICP1 genome replication, thus preventing ICP1 plaquing. We further characterize the conflict between DarTG-mediated defense and ICP1 by identifying an ICP1-encoded protein that counters DarTG and allows ICP1 progeny production. Finally, we identify this protein, AdfB, as a functional antitoxin that abrogates the toxin DarT likely through direct interactions. Following the detection of the DarTG system in clinical V. cholerae isolates, we observed a rise in ICP1 isolates with the functional antitoxin. These data highlight the use of surveillance of V. cholerae and its lytic phages to understand the co-evolutionary arms race between bacteria and their phages in nature.IMPORTANCEThe global bacterial pathogen Vibrio cholerae causes an estimated 1 to 4 million cases of cholera each year. Thus, studying the factors that influence its persistence as a pathogen is of great importance. One such influence is the lytic phage ICP1, as once infected by ICP1, V. cholerae is destroyed. To date, we have observed that the phage ICP1 shapes V. cholerae evolution through the flux of anti-phage bacterial immune systems. Here, we probe clinical V. cholerae isolates for novel anti-phage immune systems that can inhibit ICP1 and discover the toxin-antitoxin system DarTG as a potent inhibitor. Our results underscore the importance of V. cholerae and ICP1 surveillance to elaborate novel means by which V. cholerae can persist in both the human host and aquatic reservoir in the face of ICP1.

Bacteria renew an OLD protein to cleave host tRNAs and block phage translation

Anti-phage defenses must rapidly sense and respond to diverse viruses. A recent pair of papers in Nature reveal via structural and functional assays how the PARIS defense system, a recently discovered toxin-antitoxin system, senses phage-associated molecular patterns (PhAMPs), thereby activating an endonuclease toxin that cleaves tRNA to block phage replication.

Phage-triggered reverse transcription assembles a toxic repetitive gene from a noncoding RNA

Reverse transcription has frequently been co-opted for cellular functions and in prokaryotes is associated with protection against viral infection, but the underlying mechanisms of defense are generally unknown. Here, we show that in the DRT2 defense system, the reverse transcriptase binds a neighboring pseudoknotted noncoding RNA. Upon bacteriophage infection, a template region of this RNA is reverse transcribed into an array of tandem repeats that reconstitute a promoter and open reading frame, allowing expression of a toxic repetitive protein and an abortive infection response. Biochemical reconstitution of this activity and cryo-electron microscopy provide a molecular basis for repeat synthesis. Gene synthesis from a noncoding RNA is a previously unknown mode of genetic regulation in prokaryotes.

Architecture and activation mechanism of the bacterial PARIS defence system

Bacteria and their viruses (bacteriophages or phages) are engaged in an intense evolutionary arms race1-5. While the mechanisms of many bacterial antiphage defence systems are known1, how these systems avoid toxicity outside infection yet activate quickly after infection is less well understood. Here we show that the bacterial phage anti-restriction-induced system (PARIS) operates as a toxin-antitoxin system, in which the antitoxin AriA sequesters and inactivates the toxin AriB until triggered by the T7 phage counterdefence protein Ocr. Using cryo-electron microscopy, we show that AriA is related to SMC-family ATPases but assembles into a distinctive homohexameric complex through two oligomerization interfaces. In uninfected cells, the AriA hexamer binds to up to three monomers of AriB, maintaining them in an inactive state. After Ocr binding, the AriA hexamer undergoes a structural rearrangement, releasing AriB and allowing it to dimerize and activate. AriB is a toprim/OLD-family nuclease, the activation of which arrests cell growth and inhibits phage propagation by globally inhibiting protein translation through specific cleavage of a lysine tRNA. Collectively, our findings reveal the intricate molecular mechanisms of a bacterial defence system triggered by a phage counterdefence protein, and highlight how an SMC-family ATPase has been adapted as a bacterial infection sensor.

Phages reconstitute NAD+ to counter bacterial immunity

Bacteria defend against phage infection through a variety of antiphage defence systems1. Many defence systems were recently shown to deplete cellular nicotinamide adenine dinucleotide (NAD+) in response to infection, by cleaving NAD+ into ADP-ribose (ADPR) and nicotinamide2-7. It was demonstrated that NAD+ depletion during infection deprives the phage of this essential molecule and impedes phage replication. Here we show that a substantial fraction of phages possess enzymatic pathways allowing reconstitution of NAD+ from its degradation products in infected cells. We describe NAD+ reconstitution pathway 1 (NARP1), a two-step pathway in which one enzyme phosphorylates ADPR to generate ADPR pyrophosphate (ADPR-PP), and the second enzyme conjugates ADPR-PP and nicotinamide to generate NAD+. Phages encoding NARP1 can overcome a diverse set of defence systems, including Thoeris, DSR1, DSR2, SIR2-HerA and SEFIR, all of which deplete NAD+ as part of their defensive mechanism. Phylogenetic analyses show that NARP1 is primarily encoded on phage genomes, suggesting a phage-specific function in countering bacterial defences. A second pathway, NARP2, allows phages to overcome bacterial defences by building NAD+ using metabolites different from ADPR-PP. Our findings reveal a unique immune evasion strategy in which viruses rebuild molecules depleted by defence systems, thus overcoming host immunity.

A phage satellite manipulates the viral DNA packaging motor to inhibit phage and promote satellite spread

ICP1, a lytic bacteriophage of Vibrio cholerae, is parasitized by phage satellites, PLEs, which hijack ICP1 proteins for their own horizontal spread. PLEs' dependence on ICP1's DNA replication machinery and virion components results in inhibition of ICP1's lifecycle. PLEs are expected to depend on ICP1 factors for genome packaging, but the mechanism(s) PLEs use to inhibit ICP1 genome packaging is currently unknown. Here, we identify and characterize Gpi, PLE's indiscriminate genome packaging inhibitor. Gpi binds to ICP1's large terminase (TerL), the packaging motor, and blocks genome packaging. To overcome Gpi's negative effect on TerL, a component PLE also requires, PLE uses two genome packaging specifiers, GpsA and GpsB, that specifically allow packaging of PLE genomes. Surprisingly, PLE also uses mimicry of ICP1's pac site as a backup strategy to ensure genome packaging. PLE's pac site mimicry, however, is only sufficient if PLE can inhibit ICP1 at other stages of its lifecycle, suggesting an advantage to maintaining Gpi, GpsA and GpsB. Collectively, these results provide mechanistic insights into another stage of ICP1's lifecycle that is inhibited by PLE, which is currently the most inhibitory of the documented phage satellites. More broadly, Gpi represents the first satellite-encoded inhibitor of a phage TerL.

Conservation of antiviral systems across domains of life reveals immune genes in humans

Deciphering the immune organization of eukaryotes is important for human health and for understanding ecosystems. The recent discovery of antiphage systems revealed that various eukaryotic immune proteins originate from prokaryotic antiphage systems. However, whether bacterial antiphage proteins can illuminate immune organization in eukaryotes remains unexplored. Here, we use a phylogeny-driven approach to uncover eukaryotic immune proteins by searching for homologs of bacterial antiphage systems. We demonstrate that proteins displaying sequence similarity with recently discovered antiphage systems are widespread in eukaryotes and maintain a role in human immunity. Two eukaryotic proteins of the anti-transposon piRNA pathway are evolutionarily linked to the antiphage system Mokosh. Additionally, human GTPases of immunity-associated proteins (GIMAPs) as well as two genes encoded in microsynteny, FHAD1 and CTRC, are respectively related to the Eleos and Lamassu prokaryotic systems and exhibit antiviral activity. Our work illustrates how comparative genomics of immune mechanisms can uncover defense genes in eukaryotes.

Supramolecular assemblies in bacterial immunity: an emerging paradigm

The study of bacterial immune systems has recently gained momentum, revealing a fascinating trend: many systems form large supramolecular assemblies. Here, we examine the potential mechanisms underpinning the evolutionary success of these structures, draw parallels to eukaryotic immunity, and offer fresh perspectives to stimulate future research into bacterial immunity.

Viperin immunity evolved across the tree of life through serial innovations on a conserved scaffold

Evolutionary arms races between cells and viruses drive the rapid diversification of antiviral genes in diverse life forms. Recent discoveries have revealed the existence of immune genes that are shared between prokaryotes and eukaryotes and show molecular and mechanistic similarities in their response to viruses. However, the evolutionary dynamics underlying the conservation and adaptation of these antiviral genes remain mostly unexplored. Here, we show that viperins constitute a highly conserved family of immune genes across diverse prokaryotes and eukaryotes and identify mechanisms by which they diversified in eukaryotes. Our findings indicate that viperins are enriched in Asgard archaea and widely distributed in all major eukaryotic clades, suggesting their presence in the last eukaryotic common ancestor and their acquisition in eukaryotes from an archaeal lineage. We show that viperins maintain their immune function by producing antiviral nucleotide analogues and demonstrate that eukaryotic viperins diversified through serial innovations on the viperin gene, such as the emergence and selection of substrate specificity towards pyrimidine nucleotides, and through partnerships with genes maintained through genetic linkage, notably with nucleotide kinases. These findings unveil biochemical and genomic transitions underlying the adaptation of immune genes shared by prokaryotes and eukaryotes. Our study paves the way for further understanding of the conservation of immunity across domains of life.

Tail assembly interference is a common strategy in bacterial antiviral defenses

Many bacterial immune systems recognize phage structural components to activate antiviral responses, without inhibiting the function of the phage component. These systems can be encoded in specific chromosomal loci, known as defense islands, and in mobile genetic elements such as prophages and phage-inducible chromosomal islands (PICIs). Here, we identify a family of bacterial immune systems, named Tai (for 'tail assembly inhibition'), that is prevalent in PICIs, prophages and P4-like phage satellites. Tai systems protect their bacterial host population from other phages by blocking the tail assembly step, leading to the release of tailless phages incapable of infecting new hosts. To prevent autoimmunity, some Tai-positive phages have an associated counter-defense mechanism that is expressed during the phage lytic cycle and allows for tail formation. Interestingly, the Tai defense and counter-defense genes are organized in a non-contiguous operon, enabling their coordinated expression.

The role of TIR domain-containing proteins in bacterial defense against phages

Toll/interleukin-1 receptor (TIR) domain-containing proteins play a critical role in immune responses in diverse organisms, but their function in bacterial systems remains to be fully elucidated. This study, focusing on Escherichia coli, addresses how TIR domain-containing proteins contribute to bacterial immunity against phage attack. Through an exhaustive survey of all E. coli genomes available in the NCBI database and testing of 32 representatives of the 90% of the identified TIR domain-containing proteins, we found that a significant proportion (37.5%) exhibit antiphage activities. These defense systems recognize a variety of phage components, thus providing a sophisticated mechanism for pathogen detection and defense. This study not only highlights the robustness of TIR systems in bacterial immunity, but also draws an intriguing parallel to the diversity seen in mammalian Toll-like receptors (TLRs), enriching our understanding of innate immune mechanisms across life forms and underscoring the evolutionary significance of these defense strategies in prokaryotes.

DNA polymerase swapping in Caudoviricetes bacteriophages

BACKGROUND

Viruses with double-stranded (ds) DNA genomes in the realm Duplodnaviria share a conserved structural gene module but show a broad range of variation in their repertoires of DNA replication proteins. Some of the duplodnaviruses encode (nearly) complete replication systems whereas others lack (almost) all genes required for replication, relying on the host replication machinery. DNA polymerases (DNAPs) comprise the centerpiece of the DNA replication apparatus. The replicative DNAPs are classified into 4 unrelated or distantly related families (A-D), with the protein structures and sequences within each family being, generally, highly conserved. More than half of the duplodnaviruses encode a DNAP of family A, B or C. We showed previously that multiple pairs of closely related viruses in the order Crassvirales encode DNAPs of different families.

METHODS

Groups of phages in which DNAP swapping likely occurred were identified as subtrees of a defined depth in a comprehensive evolutionary tree of tailed bacteriophages that included phages with DNAPs of different families. The DNAP swaps were validated by constrained tree analysis that was performed on phylogenetic tree of large terminase subunits, and the phage genomes encoding swapped DNAPs were aligned using Mauve. The structures of the discovered unusual DNAPs were predicted using AlphaFold2.

RESULTS

We identified four additional groups of tailed phages in the class Caudoviricetes in which the DNAPs apparently were swapped on multiple occasions, with replacements occurring both between families A and B, or A and C, or between distinct subfamilies within the same family. The DNAP swapping always occurs "in situ", without changes in the organization of the surrounding genes. In several cases, the DNAP gene is the only region of substantial divergence between closely related phage genomes, whereas in others, the swap apparently involved neighboring genes encoding other proteins involved in phage genome replication. In addition, we identified two previously undetected, highly divergent groups of family A DNAPs that are encoded in some phage genomes along with the main DNAP implicated in genome replication.

CONCLUSIONS

Replacement of the DNAP gene by one encoding a DNAP of a different family occurred on many independent occasions during the evolution of different families of tailed phages, in some cases, resulting in very closely related phages encoding unrelated DNAPs. DNAP swapping was likely driven by selection for avoidance of host antiphage mechanisms targeting the phage DNAP that remain to be identified, and/or by selection against replicon incompatibility.

Exploring the diversity of anti-defense systems across prokaryotes, phages, and mobile genetic elements

The co-evolution of prokaryotes, phages, and mobile genetic elements (MGEs) over the past billions of years has driven the emergence and diversification of defense and anti-defense systems alike. Anti-defense proteins have diverse functional domains, sequences, and are typically small, creating a challenge to detect anti-defense homologs across the prokaryotic genomes. To date, no tools comprehensively annotate anti-defense proteins within a desired genome or MGE. Here, we developed "AntiDefenseFinder" - a free open-source tool and web service that detects 156 anti-defense systems (of one or more proteins) in any genomic sequence. Using this dataset, we identified 47,981 anti-defense systems distributed across prokaryotes, phage, and MGEs. We found that some genes co-localize in "anti-defense islands", including E. coli T4 and Lambda phages, although many are standalone. Out of the 112 systems detected in bacteria, 100 systems localize only or preferentially in prophages, plasmids, phage satellites, integrons, and integrative and conjugative elements. However, over 80% of anti-Pycsar protein 1 (Apyc1) resides in non-mobile regions of bacteria. Evolutionary and functional analyses revealed that Apyc1 likely originated in bacteria to regulate cNMP signaling, but was co-opted multiple times by phages to overcome cNMP-utilizing defenses. With the AntiDefenseFinder tool, we hope to facilitate the identification of the full repertoire of anti-defense systems in MGEs, the discovery of new protein functions, and a deeper understanding of host-pathogen arms race.