Multiple phage resistance systems inhibit infection via SIR2-dependent NAD+ depletion

Bacteriophage cocktail for biocontrol of soft rot disease caused by Pectobacterium species in Chinese cabbage

Pectobacterium spp. are necrotrophic plant pathogens that cause the soft rot disease in Chinese cabbage, resulting in severe yield loss. The use of conventional antimicrobial agents, copper-based bactericides, and antibiotics has encountered several limitations, such as bioaccumulation on plants and microbial resistance. Bacteriophages (phages) are considered promising alternative antimicrobial agents against diverse phytopathogens. In this study, we isolated and characterized two virulent phages (phiPccP-2 and phiPccP-3) to develop a phage cocktail. Morphological and genomic analyses revealed that two phages belonged to the Tevenvirinae and Mccorquodalevirinae subfamilies, respectively. The phiPccP-2 and phiPccP-3 phages, which have a broad host range, were stable at various environmental conditions, such as various pHs and temperatures and exposure to ultraviolet light. The phage cocktail developed using these two lytic phages inhibited the emergence of phage-resistant bacteria compared to single-phage treatments in in vitro challenge assays. The phage cocktail treatment effectively prevented the development of soft rot symptom in matured Chinese cabbage leaves. Additionally, the phage cocktail comprising three phages (phiPccP-1, phiPccP-2, and phiPccP-3) showed superior biocontrol efficacy against the mixture of Pectobacterium strains in Chinese cabbage seedlings. These results suggest that developing phage cocktails is an effective approach for biocontrol of soft rot disease caused by Pectobacterium strains in crops compared to single-phage treatments. KEY POINTS: •Two newly isolated Pectobacterium phages, phiPccP-2 and phiPccP-3, infected diverse Pectobacterium species and effectively inhibited the emergence of phage-resistant bacteria. •Genomic and physiological analyses suggested that both phiPccP-2 and phiPccP-3 are lytic phages and that their lytic activities are stable in the environmental conditions under which Chinese cabbage grows. •Treatment using a phage cocktail comprising phiPccP-2 and phiPccP-3 efficiently suppressed soft rot disease in detached mature leaves and seedlings of Chinese cabbage, indicating the applicability of the phage cocktail as an alternative antimicrobial agent.

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.

Discovery of antiphage systems in the lactococcal plasmidome

Until the late 2000s, lactococci substantially contributed to the discovery of various plasmid-borne phage defence systems, rendering these bacteria an excellent antiphage discovery resource. Recently, there has been a resurgence of interest in identifying novel antiphage systems in lactic acid bacteria owing to recent reports of so-called 'defence islands' in diverse bacterial genera. Here, 321 plasmid sequences from 53 lactococcal strains were scrutinized for the presence of antiphage systems. Systematic evaluation of 198 candidates facilitated the discovery of seven not previously described antiphage systems, as well as five systems, of which homologues had been described in other bacteria. All described systems confer resistance against the most prevalent lactococcal phages, and act post phage DNA injection, while all except one behave like abortive infection systems. Structure and domain predictions provided insights into their mechanism of action and allow grouping of several genetically distinct systems. Although rare within our plasmid collection, homologues of the seven novel systems appear to be widespread among bacteria. This study highlights plasmids as a rich repository of as yet undiscovered antiphage systems.

Structural insights into activation mechanisms on NADase of the bacterial DSR2 anti-phage defense system

As a sirtuin (SIR2) family protein, defense-associated sirtuin2 (DSR2) has been demonstrated to participate in bacterial anti-phage resistance via depleting nicotinamide adenine dinucleotide (NAD+) of infected cells, which can be activated by tail tube protein (TTP) and inhibited by DSR anti-defense 1 (DSAD1) of diverse phages. However, the regulating mechanism remains elusive. Here, we determined the cryo-electron microscopy structure of apo DSR2, as well as the respective complex structures with TTP and DSAD1. Structural analyses and biochemical studies reveal that DSR2 forms a tetramer with a SIR2 central core and two distinct conformations. Monomeric TTP preferentially binds to the closed conformation of DSR2, inducing conformational distortions on SIR2 tetramer assembly to activate its NADase activity. DSAD1 combines with the open conformation of DSR2, directly or allosterically inhibiting TTP activation on DSR2 NAD+ hydrolysis. Our findings decipher the detailed molecule mechanisms for DSR2 NADase activity regulation and lay a foundation for in-depth understanding of the DSR2 anti-phage defense system.

Molecular and structural basis of an ATPase-nuclease dual-enzyme anti-phage defense complex

Coupling distinct enzymatic effectors emerges as an efficient strategy for defense against phage infection in bacterial immune responses, such as the widely studied nuclease and cyclase activities in the type III CRISPR-Cas system. However, concerted enzymatic activities in other bacterial defense systems are poorly understood. Here, we biochemically and structurally characterize a two-component defense system DUF4297-HerA, demonstrating that DUF4297-HerA confers resistance against phage infection by cooperatively cleaving dsDNA and hydrolyzing ATP. DUF4297 alone forms a dimer, and HerA alone exists as a nonplanar split spiral hexamer, both of which exhibit extremely low enzymatic activity. Interestingly, DUF4297 and HerA assemble into an approximately 1 MDa supramolecular complex, where two layers of DUF4297 (6 DUF4297 molecules per layer) linked via inter-layer dimerization of neighboring DUF4297 molecules are stacked on top of the HerA hexamer. Importantly, the complex assembly promotes dimerization of DUF4297 molecules in the upper layer and enables a transition of HerA from a nonplanar hexamer to a planar hexamer, thus activating their respective enzymatic activities to abrogate phage infection. Together, our findings not only characterize a novel dual-enzyme anti-phage defense system, but also reveal a unique activation mechanism by cooperative complex assembly in bacterial immunity.

Structural insights into autoinhibition and activation of defense-associated sirtuin protein

Bacterial defense-associated sirtuin 2 (DSR2) proteins harbor an N-terminal sirtuin (SIR2) domain degrading NAD+. DSR2 from Bacillus subtilis 29R is autoinhibited and unable to hydrolyze NAD+ in the absence of phage infection. A tail tube protein (TTP) of phage SPR activates the DSR2 while a DSR2-inhibiting protein of phage SPbeta, known as DSAD1 (DSR anti-defense 1), inactivates the DSR2. Although DSR2 structures in complexed with TTP and DSAD1, respectively, have been reported recently, the autoinhibition and activation mechanisms remain incompletely understood. Here, we present cryo-electron microscopy structures of the DSR2-NAD+ complex in autoinhibited state and the in vitro assembled DSR2-TFD (TTP tube-forming domain) complex in activated state. The DSR2-NAD+ complex reveals that the autoinhibited DSR2 assembles into an inactive tetramer, binding NAD+ through a distinct pocket situated outside active site. Binding of TFD into cavities within the sensor domains of DSR2 triggers a conformational change in SIR2 regions, activating its NADase activity, whereas the TTP β-sandwich domain (BSD) is flexible and does not contribute to the activation process. The activated form of DSR2 exists as tetramers and dimers, with the tetramers exhibiting more NADase activity. Overall, our results extend the current understanding of autoinhibition and activation of DSR2 immune proteins.

The structural basis of the activation and inhibition of DSR2 NADase by phage proteins

DSR2, a Sir2 domain-containing protein, protects bacteria from phage infection by hydrolyzing NAD+. The enzymatic activity of DSR2 is triggered by the SPR phage tail tube protein (TTP), while suppressed by the SPbeta phage-encoded DSAD1 protein, enabling phages to evade the host defense. However, the molecular mechanisms of activation and inhibition of DSR2 remain elusive. Here, we report the cryo-EM structures of apo DSR2, DSR2-TTP-NAD+ and DSR2-DSAD1 complexes. DSR2 assembles into a head-to-head tetramer mediated by its Sir2 domain. The C-terminal helical regions of DSR2 constitute four partner-binding cavities with opened and closed conformation. Two TTP molecules bind to two of the four C-terminal cavities, inducing conformational change of Sir2 domain to activate DSR2. Furthermore, DSAD1 competes with the activator for binding to the C-terminal cavity of DSR2, effectively suppressing its enzymatic activity. Our results provide the mechanistic insights into the DSR2-mediated anti-phage defense system and DSAD1-dependent phage immune evasion.

Isolation and Characterization of Two Novel Genera of Jumbo Bacteriophages Infecting Xanthomonas vesicatoria Isolated from Agricultural Regions in Mexico

Bacterial spot is a serious disease caused by several species of Xanthomonas affecting pepper and tomato production worldwide. Since the strategies employed for disease management have been inefficient and pose a threat for environmental and human health, the development of alternative methods is gaining relevance. The aim of this study is to isolate and characterize lytic phages against Xanthomonas pathogens. Here, we isolate two jumbo phages, named XaC1 and XbC2, from water obtained from agricultural irrigation channels by the enrichment technique using X. vesicatoria as a host. We determined that both phages were specific for inducing the lysis of X. vesicatoria strains, but not of other xanthomonads. The XaC1 and XbC2 phages showed a myovirus morphology and were classified as jumbo phages due to their genomes being larger than 200 kb. Phylogenetic and comparative analysis suggests that XaC1 and XbC2 represent both different and novel genera of phages, where XaC1 possesses a low similarity to other phage genomes reported before. Finally, XaC1 and XbC2 exhibited thermal stability up to 45 °C and pH stability from 5 to 9. All these results indicate that the isolated phages are promising candidates for the development of formulations against bacterial spot, although further characterization is required.

Conservation and similarity of bacterial and eukaryotic innate immunity

Pathogens are ubiquitous and a constant threat to their hosts, which has led to the evolution of sophisticated immune systems in bacteria, archaea and eukaryotes. Bacterial immune systems encode an astoundingly large array of antiviral (antiphage) systems, and recent investigations have identified unexpected similarities between the immune systems of bacteria and animals. In this Review, we discuss advances in our understanding of the bacterial innate immune system and highlight the components, strategies and pathogen restriction mechanisms conserved between bacteria and eukaryotes. We summarize evidence for the hypothesis that components of the human immune system originated in bacteria, where they first evolved to defend against phages. Further, we discuss shared mechanisms that pathogens use to overcome host immune pathways and unexpected similarities between bacterial immune systems and interbacterial antagonism. Understanding the shared evolutionary path of immune components across domains of life and the successful strategies that organisms have arrived at to restrict their pathogens will enable future development of therapeutics that activate the human immune system for the precise treatment of disease.

Structural characterization of macro domain-containing Thoeris antiphage defense systems

Thoeris defense systems protect bacteria from infection by phages via abortive infection. In these systems, ThsB proteins serve as sensors of infection and generate signaling nucleotides that activate ThsA effectors. Silent information regulator and SMF/DprA-LOG (SIR2-SLOG) containing ThsA effectors are activated by cyclic ADP-ribose (ADPR) isomers 2'cADPR and 3'cADPR, triggering abortive infection via nicotinamide adenine dinucleotide (NAD+) depletion. Here, we characterize Thoeris systems with transmembrane and macro domain (TM-macro)-containing ThsA effectors. We demonstrate that ThsA macro domains bind ADPR and imidazole adenine dinucleotide (IAD), but not 2'cADPR or 3'cADPR. Combining crystallography, in silico predictions, and site-directed mutagenesis, we show that ThsA macro domains form nucleotide-induced higher-order oligomers, enabling TM domain clustering. We demonstrate that ThsB can produce both ADPR and IAD, and we identify a ThsA TM-macro-specific ThsB subfamily with an active site resembling deoxy-nucleotide and deoxy-nucleoside processing enzymes. Collectively, our study demonstrates that Thoeris systems with SIR2-SLOG and TM-macro ThsA effectors trigger abortive infection via distinct mechanisms.

A Vibrio cholerae Anti-Phage System Depletes Nicotinamide Adenine Dinucleotide to Restrict Virulent Bacteriophages

Bacteria and their predatory viruses (bacteriophages or phages) are in a perpetual molecular arms race. This has led to the evolution of numerous phage defensive systems in bacteria that are still being discovered, as well as numerous ways of interference or circumvention on the part of phages. Here, we identify a unique molecular battle between the classical biotype of Vibrio cholerae and virulent phages ICP1, ICP2, and ICP3. We show that classical biotype strains resist almost all isolates of these phages due to a 25-kb genomic island harboring several putative anti-phage systems. We observed that one of these systems, Nezha, encoding SIR2-like and helicase proteins, inhibited the replication of all three phages. Bacterial SIR2-like enzymes degrade the essential metabolic coenzyme nicotinamide adenine dinucleotide (NAD+), thereby preventing replication of the invading phage. In support of this mechanism, we identified one phage isolate, ICP1_2001, which circumvents Nezha by encoding two putative NAD+ regeneration enzymes. By restoring the NAD+ pool, we hypothesize that this system antagonizes Nezha without directly interacting with either protein and should be able to antagonize other anti-phage systems that deplete NAD+.

Going viral: The role of mobile genetic elements in bacterial immunity

Bacteriophages and other mobile genetic elements (MGEs) pose a significant threat to bacteria, subjecting them to constant attacks. In response, bacteria have evolved a sophisticated immune system that employs diverse defensive strategies and mechanisms. Remarkably, a growing body of evidence suggests that most of these defenses are encoded by MGEs themselves. This realization challenges our traditional understanding of bacterial immunity and raises intriguing questions about the evolutionary forces at play. Our review provides a comprehensive overview of the latest findings on the main families of MGEs and the defense systems they encode. We also highlight how a vast diversity of defense systems remains to be discovered and their mechanism of mobility understood. Altogether, the composition and distribution of defense systems in bacterial genomes only makes sense in the light of the ecological and evolutionary interactions of a complex network of MGEs.

Retron-Eco1 assembles NAD+-hydrolyzing filaments that provide immunity against bacteriophages

Retrons are toxin-antitoxin systems protecting bacteria against bacteriophages via abortive infection. The Retron-Eco1 antitoxin is formed by a reverse transcriptase (RT) and a non-coding RNA (ncRNA)/multi-copy single-stranded DNA (msDNA) hybrid that neutralizes an uncharacterized toxic effector. Yet, the molecular mechanisms underlying phage defense remain unknown. Here, we show that the N-glycosidase effector, which belongs to the STIR superfamily, hydrolyzes NAD+ during infection. Cryoelectron microscopy (cryo-EM) analysis shows that the msDNA stabilizes a filament that cages the effector in a low-activity state in which ADPr, a NAD+ hydrolysis product, is covalently linked to the catalytic E106 residue. Mutations shortening the msDNA induce filament disassembly and the effector's toxicity, underscoring the msDNA role in immunity. Furthermore, we discovered a phage-encoded Retron-Eco1 inhibitor (U56) that binds ADPr, highlighting the intricate interplay between retron systems and phage evolution. Our work outlines the structural basis of Retron-Eco1 defense, uncovering ADPr's pivotal role in immunity.

Molecular basis of bacterial DSR2 anti-phage defense and viral immune evasion

Defense-associated sirtuin 2 (DSR2) systems are widely distributed across prokaryotic genomes, providing robust protection against phage infection. DSR2 recognizes phage tail tube proteins and induces abortive infection by depleting intracellular NAD+, a process that is counteracted by another phage-encoded protein, DSR Anti Defense 1 (DSAD1). Here, we present cryo-EM structures of Bacillus subtilis DSR2 in its apo, Tube-bound, and DSAD1-bound states. DSR2 assembles into an elongated tetramer, with four NADase catalytic modules clustered in the center and the regulatory-sensing modules distributed at four distal corners. Interestingly, monomeric Tube protein, rather than its oligomeric states, docks at each corner of the DSR2 tetramer to form a 4:4 DSR2-Tube assembly, which is essential for DSR2 NADase activity. DSAD1 competes with Tube for binding to DSR2 by occupying an overlapping region, thereby inhibiting DSR2 immunity. Thus, our results provide important insights into the assembly, activation and inhibition of the DSR2 anti-phage defense system.

Bacteriophages avoid autoimmunity from cognate immune systems as an intrinsic part of their life cycles

Dormant prophages protect lysogenic cells by expressing diverse immune systems, which must avoid targeting their cognate prophages upon activation. Here we report that multiple Staphylococcus aureus prophages encode Tha (tail-activated, HEPN (higher eukaryotes and prokaryotes nucleotide-binding) domain-containing anti-phage system), a defence system activated by structural tail proteins of incoming phages. We demonstrate the function of two Tha systems, Tha-1 and Tha-2, activated by distinct tail proteins. Interestingly, Tha systems can also block reproduction of the induced tha-positive prophages. To prevent autoimmunity after prophage induction, these systems are inhibited by the product of a small overlapping antisense gene previously believed to encode an excisionase. This genetic organization, conserved in S. aureus prophages, allows Tha systems to protect prophages and their bacterial hosts against phage predation and to be turned off during prophage induction, balancing immunity and autoimmunity. Our results show that the fine regulation of these processes is essential for the correct development of prophages' life cycle.

DNA-targeting short Argonautes complex with effector proteins for collateral nuclease activity and bacterial population immunity

Two prokaryotic defence systems, prokaryotic Argonautes (pAgos) and CRISPR-Cas, detect and cleave invader nucleic acids using complementary guides and the nuclease activities of pAgo or Cas proteins. However, not all pAgos are active nucleases. A large clade of short pAgos bind nucleic acid guides but lack nuclease activity, suggesting a different mechanism of action. Here we investigate short pAgos associated with a putative effector nuclease, NbaAgo from Novosphingopyxis baekryungensis and CmeAgo from Cupriavidus metallidurans. We show that these pAgos form a heterodimeric complex with co-encoded effector nucleases (short prokaryotic Argonaute, DNase and RNase associated (SPARDA)). RNA-guided target DNA recognition unleashes the nuclease activity of SPARDA leading to indiscriminate collateral cleavage of DNA and RNA. Activation of SPARDA by plasmids or phages results in degradation of cellular DNA and cell death or dormancy, conferring target-specific population protection and expanding the range of known prokaryotic immune systems.

Knowing Our Enemy in the Antimicrobial Resistance Era: Dissecting the Molecular Basis of Bacterial Defense Systems

Bacteria and their phage adversaries are engaged in an ongoing arms race, resulting in the development of a broad antiphage arsenal and corresponding viral countermeasures. In recent years, the identification and utilization of CRISPR-Cas systems have driven a renewed interest in discovering and characterizing antiphage mechanisms, revealing a richer diversity than initially anticipated. Currently, these defense systems can be categorized based on the bacteria's strategy associated with the infection cycle stage. Thus, bacterial defense systems can degrade the invading genetic material, trigger an abortive infection, or inhibit genome replication. Understanding the molecular mechanisms of processes related to bacterial immunity has significant implications for phage-based therapies and the development of new biotechnological tools. This review aims to comprehensively cover these processes, with a focus on the most recent discoveries.

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.

Inhibitors of bacterial immune systems: discovery, mechanisms and applications

To contend with the diversity and ubiquity of bacteriophages and other mobile genetic elements, bacteria have developed an arsenal of immune defence mechanisms. Bacterial defences include CRISPR-Cas, restriction-modification and a growing list of mechanistically diverse systems, which constitute the bacterial 'immune system'. As a response, bacteriophages and mobile genetic elements have evolved direct and indirect mechanisms to circumvent or block bacterial defence pathways and ensure successful infection. Recent advances in methodological and computational approaches, as well as the increasing availability of genome sequences, have boosted the discovery of direct inhibitors of bacterial defence systems. In this Review, we discuss methods for the discovery of direct inhibitors, their diverse mechanisms of action and perspectives on their emerging applications in biotechnology and beyond.

Auto-inhibition and activation of a short Argonaute-associated TIR-APAZ defense system

Short prokaryotic Ago accounts for most prokaryotic Argonaute proteins (pAgos) and is involved in defending bacteria against invading nucleic acids. Short pAgo associated with TIR-APAZ (SPARTA) has been shown to oligomerize and deplete NAD+ upon guide-mediated target DNA recognition. However, the molecular basis of SPARTA inhibition and activation remains unknown. In this study, we determined the cryogenic electron microscopy structures of Crenotalea thermophila SPARTA in its inhibited, transient and activated states. The SPARTA monomer is auto-inhibited by its acidic tail, which occupies the guide-target binding channel. Guide-mediated target binding expels this acidic tail and triggers substantial conformational changes to expose the Ago-Ago dimerization interface. As a result, SPARTA assembles into an active tetramer, where the four TIR domains are rearranged and packed to form NADase active sites. Together with biochemical evidence, our results provide a panoramic vision explaining SPARTA auto-inhibition and activation and expand understanding of pAgo-mediated bacterial defense systems.

Multi-layered genome defences in bacteria

Bacteria have evolved a variety of defence mechanisms to protect against mobile genetic elements, including restriction-modification systems and CRISPR-Cas. In recent years, dozens of previously unknown defence systems (DSs) have been discovered. Notably, diverse DSs often coexist within the same genome, and some co-occur at frequencies significantly higher than would be expected by chance, implying potential synergistic interactions. Recent studies have provided evidence of defence mechanisms that enhance or complement one another. Here, we review the interactions between DSs at the mechanistic, regulatory, ecological and evolutionary levels.

Structural basis for phage-mediated activation and repression of bacterial DSR2 anti-phage defense system

Silent information regulator 2 (Sir2) proteins typically catalyze NAD+-dependent protein deacetylation. The recently identified bacterial Sir2 domain-containing protein, defense-associated sirtuin 2 (DSR2), recognizes the phage tail tube and depletes NAD+ to abort phage propagation, which is counteracted by the phage-encoded DSR anti-defense 1 (DSAD1), but their molecular mechanisms remain unclear. Here, we determine cryo-EM structures of inactive DSR2 in its apo form, DSR2-DSAD1 and DSR2-DSAD1-NAD+, as well as active DSR2-tube and DSR2-tube-NAD+ complexes. DSR2 forms a tetramer with its C-terminal sensor domains (CTDs) in two distinct conformations: CTDclosed or CTDopen. Monomeric, rather than oligomeric, tail tube proteins preferentially bind to CTDclosed and activate Sir2 for NAD+ hydrolysis. DSAD1 binding to CTDopen allosterically inhibits tube binding and tube-mediated DSR2 activation. Our findings provide mechanistic insight into DSR2 assembly, tube-mediated DSR2 activation, and DSAD1-mediated inhibition and NAD+ substrate catalysis in bacterial DSR2 anti-phage defense systems.

The missing part: the Archaeoglobus fulgidus Argonaute forms a functional heterodimer with an N-L1-L2 domain protein

Argonaute (Ago) proteins are present in all three domains of life (bacteria, archaea and eukaryotes). They use small (15-30 nucleotides) oligonucleotide guides to bind complementary nucleic acid targets and are responsible for gene expression regulation, mobile genome element silencing, and defence against viruses or plasmids. According to their domain organization, Agos are divided into long and short Agos. Long Agos found in prokaryotes (long-A and long-B pAgos) and eukaryotes (eAgos) comprise four major functional domains (N, PAZ, MID and PIWI) and two structural linker domains L1 and L2. The majority (∼60%) of pAgos are short pAgos, containing only the MID and inactive PIWI domains. Here we focus on the prokaryotic Argonaute AfAgo from Archaeoglobus fulgidus DSM4304. Although phylogenetically classified as a long-B pAgo, AfAgo contains only MID and catalytically inactive PIWI domains, akin to short pAgos. We show that AfAgo forms a heterodimeric complex with a protein encoded upstream in the same operon, which is a structural equivalent of the N-L1-L2 domains of long pAgos. This complex, structurally equivalent to a long PAZ-less pAgo, outperforms standalone AfAgo in guide RNA-mediated target DNA binding. Our findings provide a missing piece to one of the first and the most studied pAgos.

Activation of Thoeris antiviral system via SIR2 effector filament assembly

To survive bacteriophage (phage) infections, bacteria developed numerous anti-phage defence systems1-7. Some of them (for example, type III CRISPR-Cas, CBASS, Pycsar and Thoeris) consist of two modules: a sensor responsible for infection recognition and an effector that stops viral replication by destroying key cellular components8-12. In the Thoeris system, a Toll/interleukin-1 receptor (TIR)-domain protein, ThsB, acts as a sensor that synthesizes an isomer of cyclic ADP ribose, 1''-3' glycocyclic ADP ribose (gcADPR), which is bound in the Smf/DprA-LOG (SLOG) domain of the ThsA effector and activates the silent information regulator 2 (SIR2)-domain-mediated hydrolysis of a key cell metabolite, NAD+ (refs. 12-14). Although the structure of ThsA has been solved15, the ThsA activation mechanism remained incompletely understood. Here we show that 1''-3' gcADPR, synthesized in vitro by the dimeric ThsB' protein, binds to the ThsA SLOG domain, thereby activating ThsA by triggering helical filament assembly of ThsA tetramers. The cryogenic electron microscopy (cryo-EM) structure of activated ThsA revealed that filament assembly stabilizes the active conformation of the ThsA SIR2 domain, enabling rapid NAD+ depletion. Furthermore, we demonstrate that filament formation enables a switch-like response of ThsA to the 1''-3' gcADPR signal.

Insertion sequence transposition activates antimycobacteriophage immunity through an lsr2-silenced lipid metabolism gene island

Insertion sequences (ISs) exist widely in bacterial genomes, but their roles in the evolution of bacterial antiphage defense remain to be clarified. Here, we report that, under the pressure of phage infection, the IS1096 transposition of Mycobacterium smegmatis into the lsr2 gene can occur at high frequencies, which endows the mutant mycobacterium with a broad-spectrum antiphage ability. Lsr2 functions as a negative regulator and directly silences expression of a gene island composed of 11 lipid metabolism-related genes. The complete or partial loss of the gene island leads to a significant decrease of bacteriophage adsorption to the mycobacterium, thus defending against phage infection. Strikingly, a phage that has evolved mutations in two tail-filament genes can re-escape from the lsr2 inactivation-triggered host defense. This study uncovered a new signaling pathway for activating antimycobacteriophage immunity by IS transposition and provided insight into the natural evolution of bacterial antiphage defense.

Anti-phage defence through inhibition of virion assembly

Bacteria have evolved diverse antiviral defence mechanisms to protect themselves against phage infection. Phages integrated into bacterial chromosomes, known as prophages, also encode defences that protect the bacterial hosts in which they reside. Here, we identify a type of anti-phage defence that interferes with the virion assembly pathway of invading phages. The protein that mediates this defence, which we call Tab (for 'Tail assembly blocker'), is constitutively expressed from a Pseudomonas aeruginosa prophage. Tab allows the invading phage replication cycle to proceed, but blocks assembly of the phage tail, thus preventing formation of infectious virions. While the infected cell dies through the activity of the replicating phage lysis proteins, there is no release of infectious phage progeny, and the bacterial community is thereby protected from a phage epidemic. Prophages expressing Tab are not inhibited during their own lytic cycle because they express a counter-defence protein that interferes with Tab function. Thus, our work reveals an anti-phage defence that operates by blocking virion assembly, thereby both preventing formation of phage progeny and allowing destruction of the infected cell due to expression of phage lysis genes.

Posttranslational modifications in bacteria during phage infection

During phage infection, both virus and bacteria attempt to gain and/or maintain control over critical bacterial functions, through a plethora of strategies. These strategies include posttranslational modifications (PTMs, including phosphorylation, ribosylation, and acetylation), as rapid and dynamic regulators of protein behavior. However, to date, knowledge on the topic remains scarce and fragmented, while a more systematic investigation lies within reach. The release of AlphaFold, which advances PTM enzyme discovery and functional elucidation, and the increasing inclusivity and scale of mass spectrometry applications to new PTM types, could significantly accelerate research in the field. In this review, we highlight the current knowledge on PTMs during phage infection, and conceive a possible pipeline for future research, following an enzyme-target-function scheme.

Biological and bioinformatic tools for the discovery of unknown phage-host combinations

The field of microbial ecology has been transformed by metagenomics in recent decades and has culminated in vast datasets that facilitate the bioinformatic dissection of complex microbial communities. Recently, attention has turned from defining the microbiota composition to the interactions and relationships that occur between members of the microbiota. Within complex microbiota, the identification of bacteriophage-host combinations has been a major challenge. Recent developments in artificial intelligence tools to predict protein structure and function as well as the relationships between bacteria and their infecting bacteriophages allow a strategic approach to identifying and validating phage-host relationships. However, biological validation of these predictions remains essential and will serve to improve the existing predictive tools. In this review, I provide an overview of the most recent developments in both bioinformatic and experimental approaches to predicting and experimentally validating unknown phage-host combinations.

The nuclease-associated short prokaryotic Argonaute system nonspecifically degrades DNA upon activation by target recognition

Prokaryotic Argonautes (pAgos) play a vital role in host defense by utilizing short nucleic acid guides to recognize and target complementary nucleic acids. Despite being the majority of pAgos, short pAgos have only recently received attention. Short pAgos are often associated with proteins containing an APAZ domain and a nuclease domain including DUF4365, SMEK, or HNH domain. In contrast to long pAgos that specifically cleave the target DNA, our study demonstrates that the short pAgo from Thermocrispum municipal, along with its associated DUF4365-APAZ protein, forms a heterodimeric complex. Upon RNA-guided target DNA recognition, this complex is activated to nonspecifically cleave DNA. Additionally, we found that the TmuRE-Ago complex shows a preference for 5'-OH guide RNA, specifically requires a uridine nucleotide at the 5' end of the guide RNA, and is sensitive to single-nucleotide mismatches between the guide RNA and target DNA. Based on its catalytic properties, our study has established a novel nucleic acid detection method and demonstrated its feasibility. This study not only expands our understanding of the defense mechanism employed by short pAgo systems but also suggests their potential applications in nucleic acid detection.

Correlation of Pseudomonas aeruginosa Phage Resistance with the Numbers and Types of Antiphage Systems

Phage therapeutics offer a potentially powerful approach for combating multidrug-resistant bacterial infections. However, to be effective, phage therapy must overcome existing and developing phage resistance. While phage cocktails can reduce this risk by targeting multiple receptors in a single therapeutic, bacteria have mechanisms of resistance beyond receptor modification. A rapidly growing body of knowledge describes a broad and varied arsenal of antiphage systems encoded by bacteria to counter phage infection. We sought to understand the types and frequencies of antiphage systems present in a highly diverse panel of Pseudomonas aeruginosa clinical isolates utilized to characterize novel antibacterials. Using the web-server tool PADLOC (prokaryotic antiviral defense locator), putative antiphage systems were identified in these P. aeruginosa clinical isolates based on sequence homology to a validated and curated catalog of known defense systems. Coupling this host bacterium sequence analysis with host range data for 70 phages, we observed a correlation between existing phage resistance and the presence of higher numbers of antiphage systems in bacterial genomes. We were also able to identify antiphage systems that were more prevalent in highly phage-resistant P. aeruginosa strains, suggesting their importance in conferring resistance.

Structural basis of antiphage immunity generated by a prokaryotic Argonaute-associated SPARSA system

Argonaute (Ago) proteins are ubiquitous across all kingdoms of life. Eukaryotic Agos (eAgos) use small RNAs to recognize transcripts for RNA silencing in eukaryotes. In contrast, the functions of prokaryotic counterparts (pAgo) are less well known. Recently, short pAgos in conjunction with the associated TIR or Sir2 (SPARTA or SPARSA) were found to serve as antiviral systems to combat phage infections. Herein, we present the cryo-EM structures of nicotinamide adenine dinucleotide (NAD+)-bound SPARSA with and without nucleic acids at resolutions of 3.1 Å and 3.6 Å, respectively. Our results reveal that the APAZ (Analogue of PAZ) domain and the short pAgo form a featured architecture similar to the long pAgo to accommodate nucleic acids. We further identified the key residues for NAD+ binding and elucidated the structural basis for guide RNA and target DNA recognition. Using structural comparisons, molecular dynamics simulations, and biochemical experiments, we proposed a putative mechanism for NAD+ hydrolysis in which an H186 loop mediates nucleophilic attack by catalytic water molecules. Overall, our study provides mechanistic insight into the antiphage role of the SPARSA system.

From bacteria to biomedicine: Developing therapies exploiting NAD+ metabolism

Nicotinamide adenine dinucleotide (NAD+) serves as a critical cofactor in cellular metabolism and redox reactions. Bacterial pathways rely on NAD+ participation, where its stability and concentration govern essential homeostasis and functions. This review delves into the role and metabolic regulation of NAD+ in bacteria, highlighting its influence on physiology and virulence. Notably, we explore enzymes linked to NAD+ metabolism as antibacterial drug targets and vaccine candidates. Moreover, we scrutinize NAD+'s medical potential, offering insights for its application in biomedicine. This comprehensive assessment informs future research directions in the dynamic realm of NAD+ and its biomedical significance.

Assembly-mediated activation of the SIR2-HerA supramolecular complex for anti-phage defense

SIR2-HerA, a bacterial two-protein anti-phage defense system, induces bacterial death by depleting NAD+ upon phage infection. Biochemical reconstitution of SIR2, HerA, and the SIR2-HerA complex reveals a dynamic assembly process. Unlike other ATPases, HerA can form various oligomers, ranging from dimers to nonamers. When assembled with SIR2, HerA forms a hexamer and converts SIR2 from a nuclease to an NAD+ hydrolase, representing an unexpected regulatory mechanism mediated by protein assembly. Furthermore, high concentrations of ATP can inhibit NAD+ hydrolysis by the SIR2-HerA complex. Cryo-EM structures of the SIR2-HerA complex reveal a giant supramolecular assembly up to 1 MDa, with SIR2 as a dodecamer and HerA as a hexamer, crucial for anti-phage defense. Unexpectedly, the HerA hexamer resembles a spiral staircase and exhibits helicase activities toward dual-forked DNA. Together, we reveal the supramolecular assembly of SIR2-HerA as a unique mechanism for switching enzymatic activities and bolstering anti-phage defense strategies.

Multiple enzymatic activities of a Sir2-HerA system cooperate for anti-phage defense

In response to the persistent exposure to phage infection, bacteria have evolved diverse antiviral defense mechanisms. In this study, we report a bacterial two-component defense system consisting of a Sir2 NADase and a HerA helicase. Cryo-electron microscopy reveals that Sir2 and HerA assemble into a ∼1 MDa supramolecular octadecamer. Unexpectedly, this complex exhibits various enzymatic activities, including ATPase, NADase, helicase, and nuclease, which work together in a sophisticated manner to fulfill the antiphage function. Therefore, we name this defense system "Nezha" after a divine warrior in Chinese mythology who employs multiple weapons to defeat enemies. Our findings demonstrate that Nezha could sense phage infections, self-activate to arrest cell growth, eliminate phage genomes, and subsequently deactivate to allow for cell recovery. Collectively, Nezha represents a paradigm of sophisticated and multifaceted strategies bacteria use to defend against viral infections.

Adaptation to bile and anaerobicity limits Vibrio cholerae phage adsorption

IMPORTANCE

Vibrio cholerae is the bacterial pathogen responsible for cholera, a diarrheal disease that impacts people in areas without access to potable water. In regions that lack such infrastructure, cholera represents a large proportion of disease outbreaks. Bacteriophages (phages, viruses that infect bacteria) have recently been examined as potential therapeutic and prophylactic agents to treat and prevent bacterial disease outbreaks like cholera due to their specificity and stability. This work examines the interaction between V. cholerae and vibriophages in consideration for a cholera prophylaxis regimen (M. Yen, L. S. Cairns, and A. Camilli, Nat Commun 8:14187, 2017, https://doi.org/10.1038/ncomms14187) in the context of stimuli found in the intestinal environment. We discover that common signals in the intestinal environment induce cell surface modifications in V. cholerae that also restrict some phages from binding and initiating infection. These findings could impact considerations for the design of phage-based treatments, as phage infection appears to be limited by bacterial adaptations to the intestinal environment.

Structural and functional investigation of GajB protein in Gabija anti-phage defense

Bacteriophages (phages) are viruses that infect bacteria and archaea. To fend off invading phages, the hosts have evolved a variety of anti-phage defense mechanisms. Gabija is one of the most abundant prokaryotic antiviral systems and consists of two proteins, GajA and GajB. GajA has been characterized experimentally as a sequence-specific DNA endonuclease. Although GajB was previously predicted to be a UvrD-like helicase, its function is unclear. Here, we report the results of structural and functional analyses of GajB. The crystal structure of GajB revealed a UvrD-like domain architecture, including two RecA-like core and two accessory subdomains. However, local structural elements that are important for the helicase function of UvrD are not conserved in GajB. In functional assays, GajB did not unwind or bind various types of DNA substrates. We demonstrated that GajB interacts with GajA to form a heterooctameric Gabija complex, but GajB did not exhibit helicase activity when bound to GajA. These results advance our understanding of the molecular mechanism underlying Gabija anti-phage defense and highlight the role of GajB as a component of a multi-subunit antiviral complex in bacteria.

[Bacterial immunity: Uncovering a new world]

Viruses are parasites that infect all living organisms, and bacteria are no exception. To defend themselves against their viruses (phages), bacteria have developed numerous and sophisticated defense mechanisms, our understanding of which is rapidly growing. In the 2000s, only a handful of mechanisms were known and only two of them seemed to be found in most bacteria. In 2018, a new key method based on genome analysis revealed that there were likely many others. Indeed, over the past five years, more than 150 new mechanisms have been discovered. It is now estimated that there are probably thousands. This remarkable diversity, paralleled with the tremendous viral diversity, is evident both in terms of possible combinations of systems in bacterial genomes and in molecular mechanisms. One of the most surprising observations emerging from the exploration of this diversity is the discovery of striking similarities between certain bacterial defense systems and antiviral systems in humans, as well as plant (and eukaryotes in general) immune systems. Contrary to the previously accepted paradigm, organisms as diverse as fungi, plants, bacteria and humans share certain molecular strategies to fight viral infections, suggesting that an underestimated part of eukaryotic antiviral immunity could have evolved from bacterial antiviral defense systems.

Catalytically inactive long prokaryotic Argonaute systems employ distinct effectors to confer immunity via abortive infection

Argonaute proteins (Agos) bind short nucleic acids as guides and are directed by them to recognize target complementary nucleic acids. Diverse prokaryotic Agos (pAgos) play potential functions in microbial defense. The functions and mechanisms of a group of full-length yet catalytically inactive pAgos, long-B pAgos, remain unclear. Here, we show that most long-B pAgos are functionally connected with distinct associated proteins, including nucleases, Sir2-domain-containing proteins and trans-membrane proteins, respectively. The long-B pAgo-nuclease system (BPAN) is activated by guide RNA-directed target DNA recognition and performs collateral DNA degradation in vitro. In vivo, the system mediates genomic DNA degradation after sensing invading plasmid, which kills the infected cells and results in the depletion of the invader from the cell population. Together, the BPAN system provides immunoprotection via abortive infection. Our data also suggest that the defense strategy is employed by other long-B pAgos equipped with distinct associated proteins.

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.

Abortive infection antiphage defense systems: separating mechanism and phenotype

Bacteria have evolved a wide array of mechanisms that allow them to eliminate phage infection. 'Abortive infection' (abi) systems are an expanding category of such mechanisms, defined as those which induce programmed cell death (or dormancy) upon infection, and thus halt phage propagation within a bacterial population. This definition entails two requirements - a phenotypic observation (cell death upon infection), and a mechanistic determination of its sources (system-induced death). The phenotypic and mechanistic aspects of abi are often implicitly assumed to be tightly linked, and studies regularly tend to establish one and deduce the other. However, recent evidence points to a complicated relationship between the mechanism of defense and the phenotype observed upon infection. We argue that rather than viewing the abi phenotype as an inherent quality of a set of defense systems, it should be more appropriately thought of as an attribute of interactions between specific phages and bacteria under given conditions. Consequently, we also point to potential pitfalls in the prevailing methods for ascertaining the abi phenotype. Overall, we propose an alternative framework for parsing interactions between attacking phages and defending bacteria.

CBASS to cGAS-STING: The Origins and Mechanisms of Nucleotide Second Messenger Immune Signaling

Host defense against viral pathogens is an essential function for all living organisms. In cell-intrinsic innate immunity, dedicated sensor proteins recognize molecular signatures of infection and communicate to downstream adaptor or effector proteins to activate immune defense. Remarkably, recent evidence demonstrates that much of the core machinery of innate immunity is shared across eukaryotic and prokaryotic domains of life. Here, we review a pioneering example of evolutionary conservation in innate immunity: the animal cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) signaling pathway and its ancestor in bacteria, CBASS (cyclic nucleotide-based antiphage signaling system) antiphage defense. We discuss the unique mechanism by which animal cGLRs (cGAS-like receptors) and bacterial CD-NTases (cGAS/dinucleotide-cyclase in Vibrio (DncV)-like nucleotidyltransferases) in these pathways link pathogen detection with immune activation using nucleotide second messenger signals. Comparing the biochemical, structural, and mechanistic details of cGAS-STING, cGLR signaling, and CBASS, we highlight emerging questions in the field and examine evolutionary pressures that may have shaped the origins of nucleotide second messenger signaling in antiviral defense.

Rhs NADase effectors and their immunity proteins are exchangeable mediators of inter-bacterial competition in Serratia

Many bacterial species use Type VI secretion systems (T6SSs) to deliver anti-bacterial effector proteins into neighbouring bacterial cells, representing an important mechanism of inter-bacterial competition. Specific immunity proteins protect bacteria from the toxic action of their own effectors, whilst orphan immunity proteins without a cognate effector may provide protection against incoming effectors from non-self competitors. T6SS-dependent Rhs effectors contain a variable C-terminal toxin domain (CT), with the cognate immunity protein encoded immediately downstream of the effector. Here, we demonstrate that Rhs1 effectors from two strains of Serratia marcescens, the model strain Db10 and clinical isolate SJC1036, possess distinct CTs which both display NAD(P)+ glycohydrolase activity but belong to different subgroups of NADase from each other and other T6SS-associated NADases. Comparative structural analysis identifies conserved functions required for NADase activity and reveals that unrelated NADase immunity proteins utilise a common mechanism of effector inhibition. By replicating a natural recombination event, we show successful functional exchange of CTs and demonstrate that Db10 encodes an orphan immunity protein which provides protection against T6SS-delivered SJC1036 NADase. Our findings highlight the flexible use of Rhs effectors and orphan immunity proteins during inter-strain competition and the repeated adoption of NADase toxins as weapons against bacterial cells.

Structural insights into mechanisms of Argonaute protein-associated NADase activation in bacterial immunity

Nicotinamide adenine dinucleotide (NAD+) is a central metabolite in cellular processes. Depletion of NAD+ has been demonstrated to be a prevalent theme in both prokaryotic and eukaryotic immune responses. Short prokaryotic Argonaute proteins (Agos) are associated with NADase domain-containing proteins (TIR-APAZ or SIR2-APAZ) encoded in the same operon. They confer immunity against mobile genetic elements, such as bacteriophages and plasmids, by inducing NAD+ depletion upon recognition of target nucleic acids. However, the molecular mechanisms underlying the activation of such prokaryotic NADase/Ago immune systems remain unknown. Here, we report multiple cryo-EM structures of NADase/Ago complexes from two distinct systems (TIR-APAZ/Ago and SIR2-APAZ/Ago). Target DNA binding triggers tetramerization of the TIR-APAZ/Ago complex by a cooperative self-assembly mechanism, while the heterodimeric SIR2-APAZ/Ago complex does not assemble into higher-order oligomers upon target DNA binding. However, the NADase activities of these two systems are unleashed via a similar closed-to-open transition of the catalytic pocket, albeit by different mechanisms. Furthermore, a functionally conserved sensor loop is employed to inspect the guide RNA-target DNA base pairing and facilitate the conformational rearrangement of Ago proteins required for the activation of these two systems. Overall, our study reveals the mechanistic diversity and similarity of Ago protein-associated NADase systems in prokaryotic immune response.

Prophage identification and molecular analysis in the genomes of Pseudomonas aeruginosa strains isolated from critical care patients

Prophages are bacteriophages integrated into the bacterial host's chromosome. This research aims to analyze and characterize the existing prophages within a collection of 53 Pseudomonas aeruginosa strains from intensive care units (ICUs) in Portugal and Spain. A total of 113 prophages were localized in the collection, with 18 of them being present in more than one strain simultaneously. After annotation, five of them were discarded as incomplete, and the 13 remaining prophages were characterized. Of 13, 10 belonged to the siphovirus tail morphology group, 2 to the podovirus tail morphology group, and 1 to the myovirus tail morphology group. All prophages had a length ranging from 20,199 to 63,401 bp and a GC% between 56.2% and 63.6%. The number of open reading frames (ORFs) oscillated between 32 and 88, and in 3/13 prophages, more than 50% of the ORFs had an unknown function. With our findings, we show that prophages are present in the majority of the P. aeruginosa strains isolated from Portuguese and Spanish critically ill patients, many of them found in more than one circulating strain at the same time and following a similar clonal distribution pattern. Although a great sum of ORFs had an unknown function, number of proteins in relation to viral defense (anti-CRISPR proteins, toxin/antitoxin modules, proteins against restriction-modification systems) as well as to prophage interference into their host's quorum sensing system and regulatory cascades were found. This supports the idea that prophages have an influence in bacterial pathogenesis and anti-phage defense. IMPORTANCE Despite being known for decades, prophages remain understudied when compared to the lytic phages employed in phage therapy. This research aims to shed some light into the nature, composition, and role of prophages found within a set of circulating strains of Pseudomas aeruginosa, with special attention to high-risk clones. Given the fact that prophages can effectively influence bacterial pathogenesis, prophage basic research constitutes a topic of growing interest. Furthermore, the abundance of viral defense and regulatory proteins within prophage genomes detected in this study evidences the importance of characterizing the most frequent prophages in circulating clinical strains and in high-risk clones if phage therapy is to be used.

A conserved family of immune effectors cleaves cellular ATP upon viral infection

During viral infection, cells can deploy immune strategies that deprive viruses of molecules essential for their replication. Here, we report a family of immune effectors in bacteria that, upon phage infection, degrade cellular adenosine triphosphate (ATP) and deoxyadenosine triphosphate (dATP) by cleaving the N-glycosidic bond between the adenine and sugar moieties. These ATP nucleosidase effectors are widely distributed within multiple bacterial defense systems, including cyclic oligonucleotide-based antiviral signaling systems (CBASS), prokaryotic argonautes, and nucleotide-binding leucine-rich repeat (NLR)-like proteins, and we show that ATP and dATP degradation during infection halts phage propagation. By analyzing homologs of the immune ATP nucleosidase domain, we discover and characterize Detocs, a family of bacterial defense systems with a two-component phosphotransfer-signaling architecture. The immune ATP nucleosidase domain is also encoded within diverse eukaryotic proteins with immune-like architectures, and we show biochemically that eukaryotic homologs preserve the ATP nucleosidase activity. Our findings suggest that ATP and dATP degradation is a cell-autonomous innate immune strategy conserved across the tree of life.

Defining the expanding mechanisms of phage-mediated activation of bacterial immunity

Due to recent discovery efforts, over 100 immune systems encoded by bacteria that antagonize bacteriophage (phage) replication have been uncovered. These systems employ direct and indirect mechanisms to detect phage infection and activate bacterial immunity. The most well-studied mechanisms are direct detection and activation by phage-associated molecular patterns (PhAMPs), such as phage DNA and RNA sequences, and expressed phage proteins that directly activate abortive infection systems. Phage effectors may also inhibit host processes and, therefore, indirectly activate immunity. Here, we discuss our current understanding of these protein PhAMPs and effectors expressed during various stages of the phage life cycle that activate immunity. Immune activators are predominantly identified from genetic approaches that isolate phage mutants that escape a bacterial immune system, coupled with biochemical validation. Although the mechanism of phage-mediated activation remains uncertain for most systems, it has become clear that each stage of the phage life cycle has the potential to induce a bacterial immune response.

The evolutionary success of regulated cell death in bacterial immunity

Bacteria employ a complex arsenal of immune mechanisms to defend themselves against phages. Recent studies demonstrate that these immune mechanisms frequently involve regulated cell death in response to phage infection. By sacrificing infected cells, this strategy prevents the spread of phages within the surrounding population. In this review, we discuss the principles of regulated cell death in bacterial defense, and show that over 70% of sequenced prokaryotes employ this strategy as part of their defensive arsenals. We highlight the modularity of defense systems involving regulated cell death, explaining how shuffling between phage-sensing and cell-killing protein domains dominates their evolution. Some of these defense systems are the evolutionary ancestors of key components of eukaryotic immunity, highlighting their importance in shaping the evolutionary trajectory of immune systems across the tree of life.

The arms race between bacteria CBASS and bacteriophages

The Bacterial Cyclic oligonucleotide-Based Anti-phage Signaling System (CBASS) is an innate immune system that induces cell suicide to defend against phage infections. This system relies on cGAS/DncV-like nucleotidyltransferases (CD-NTase) to synthesize cyclic oligonucleotides (cOs) and CD-NTase-associated proteins (Caps) to execute cell death through DNA cleavage, membrane damage, and NAD depletion, thereby inhibiting phage replication. Ancillary proteins expressed in CBASS, in combination with CD-NTase, ensure the normal synthesis of cOs and prepare CD-NTase for full activation by binding to phage genomes, proteins, or other unknown products. To counteract cell death induced by CBASS, phage genes encode immune evasion proteins that curb Cap recognition of cOs, allowing for phage replication, assembly, and propagation in bacterial cells. This review provides a comprehensive understanding of CBASS immunity, comparing it with different bacterial immune systems and highlighting the interplay between CBASS and phage. Additionally, it explores similar immune escape methods based on shared proteins and action mechanisms between prokaryotic and eukaryotic viruses.

A novel toolbox for precise regulation of gene expression and metabolic engineering in Bacillus licheniformis

Despite industrial bio-manufacturing progress using Bacillus licheniformis, the absence of a well-characterized toolbox allowing precise regulation of multiple genes limits its expansion for basic research and application. Here, a novel gene expression toolbox (GET) was developed for precise regulation of gene expression and high-level production of 2-phenylethanol. Firstly, we established a novel promoter core region mosaic combination model to combine, characterize and analyze different core regions. Characterization and orthogonal design of promoter ribbons allowed convenient construction of an adaptable and robust GET, gene gfp expression intensity was 0.64%-16755.77%, with a dynamic range of 2.61 × 104 times, which is the largest regulatory range of GET in Bacillus based on modification of promoter P43. Then we verified the protein and species universality of GET using different proteins expressed in B. licheniformis and Bacillus subtilis. Finally, the GET for 2-phenylethanol metabolic breeding, resulting in a plasmid-free strain producing 6.95 g/L 2-phenylethanol with a yield and productivity of 0.15 g/g glucose and 0.14 g/L/h, respectively, the highest de novo synthesis yield of 2-phenylethanol reported. Taken together, this is the first report elucidating the impact of mosaic combination and tandem of multiple core regions to initiate transcription and improve the output of proteins and metabolites, which provides strong support for gene regulation and diversified product production in Bacillus.

Conserved domains can be found across distinct phage defence systems

Bacteria are continuously exposed to predation from bacteriophages (phages) and, in response, have evolved a broad range of defence systems. These systems can prevent the replication of phages and other mobile genetic elements (MGE). Defence systems are often encoded together in genomic loci defined as "defence islands", a tendency that has been extensively exploited to identify novel antiphage systems. In the last few years, >100 new antiphage systems have been discovered, and some display homology to components of the immune systems of plants and animals. In many instances, prediction tools have found domains with similar predicted functions present as different combinations within distinct antiphage systems. In this Perspective Article, we review recent reports describing the discovery and the predicted domain composition of several novel antiphage systems. We discuss several examples of similar protein domains adopted by different antiphage systems, including domains of unknown function (DUFs), domains involved in nucleic acid recognition and degradation, and domains involved in NAD+ depletion. We further discuss the potential evolutionary advantages that could have driven the independent acquisition of these domains by different antiphage systems.