Mark Dominik Szczelkun

SauUSI is a dimeric, ATP-dependent type IV restriction enzyme that protects Staphylococcus aureus by cleaving nonself DNA containing 5-methylcytosine or 5-hydroxymethylcytosine. Using biophysical, single-molecule and nanopore sequencing methods, we show that 5-methylcytosine recognition first induces ATP-driven unidirectional translocation by one helicase-like subunit, displacing its target recognition domain (TRD). In half of events, the partner TRD can then bind the liberated modified site, stabilizing a growing DNA loop. On symmetrically methylated DNA, looping is more frequent and both motors can engage in bidirectional loop translocation. Cleavage occurs stochastically throughout translocation but events that increase dwell time such as rebinding methylated sites elevate the chance of cleavage. Interactions with other SauUSI dimers or protein roadblocks influence cleavage distributions but are not required for nuclease activation. While the first cleavages principally generate 1-nt 3ʹ overhangs or blunt ends, multiple binding–translocation–cleavage cycles by individual enzymes ultimately fragment the modified nonself DNA, neutralizing its threat.

Summary: Bacterial antiviral defense systems are highly diverse and unevenly distributed across populations, such that different strains carry distinct immune repertoires that together form a pan-immune system. Restriction-modification (RM) systems are the most abundant bacterial defense systems and exemplify this architecture, exhibiting remarkable diversity in structure, function, specificity, and efficacy. How and why such diversity is maintained, why weaker systems are not outcompeted, and whether diversity provides benefits beyond protection of individual cells remain unclear. Here, we experimentally tracked bacteria and phages across populations differing in RM diversity. We find no consistent evidence of a trade-off between defense strength and host fitness. Instead, we identify a trade-off between defense strength and horizontal transferability: stronger RM systems are less able to spread between strains, likely because factors that increase defense strength also increase toxicity risk upon transfer to unmethylated hosts. These weaker yet more mobile systems may therefore act as more accessible pan-immune components that bacterial populations can acquire in response to shifting parasite pressures. Furthermore, we show RM systems experience negative frequency-dependent selection, such that common specificities are more readily overcome by phages while rare ones remain protective, causing the benefits of potent systems to erode as they spread. Together, these dynamics maintain RM diversity and explain why weaker systems persist alongside stronger ones. Finally, we show that population-level RM diversity generates collective protection by constraining the emergence of RM-escaping phages and limiting the invasion of bacterial surface mutants that avoid phage adsorption, providing experimental support for the pan-immune model.

Bacterial defence systems are diverse and form major barriers to horizontal gene transfer and bacteriophage (phage) infection. Though restriction-modification systems were the first such defences discovered and remain the most widespread, the basis of self/non-self discrimination in many newly identified methylation-based systems remains unclear. Here, we define the mechanism of a widely distributed BREX-related defence family, previously termed Gao_RL, which we rename MANTIS (Methyltransferase, ATPases and Nuclease Targeting Inverted-repeat Sequences). Using phage infection and plasmid transformation assays in Pseudomonas aeruginosa , biochemical reconstitution, and structural biology, we show that MANTIS establishes self-identity through adenine hemimethylation of short asymmetric DNA sequences. Unexpectedly, a AAA+ ATPase (MtsA) and a co-regulator (MtsB) are both required to recruit and activate the methyltransferase MtsC, revealing a previously unknown ATP-dependent mode of epigenetic modification. DNA recognition by the MtsABC complex then triggers restriction by a dimeric helicase-nuclease effector (MtsD), with activity dependent on target-site architecture: inverted pairs of unmodified sites support robust restriction, whereas direct repeats or single sites are restricted less efficiently. Consistent with this model, virulent phages show strand-biased depletion of MANTIS target sites, indicating evolutionary pressure to evade restriction. Together, these findings reveal how dual ATPases couple self-modification to non-self restriction and provide a framework for understanding the expanding landscape of methylation-based defence systems. Graphical

SauUSI is a dimeric, ATP-dependent Type IV restriction enzyme that protects Staphylococcus aureus by cleaving non-self DNA containing 5-methylcytosine or 5-hydroxymethylcytosine. Using biophysical, single-molecule, and nanopore sequencing methods, we show that 5-methylcytosine recognition first induces ATP-driven unidirectional translocation by one helicase-like subunit, displacing its target recognition domain (TRD). The partner TRD can then bind the liberated modified site, stabilizing a growing DNA loop. On symmetrically methylated DNA, both subunits engage in bidirectional loop translocation. Cleavage is triggered by binding a distal methylated site, by preferred DNA sequences, or upon reaching a DNA end. Interactions with other SauUSI dimers or protein roadblocks affect cleavage site distributions but are not required for nuclease activation. While the first cleavages principally generate blunt-ends or one-nucleotide 3′ overhangs, multiple binding-translocation cycles by individual enzymes ultimately shred the modified non-self DNA, neutralizing its threat.

Lamassu is a diverse family of defense systems that protect bacteria, including seventh-pandemic strains of Vibrio cholerae, against both plasmids and phage infection. During phage infection, Lamassu targets essential cellular processes, thereby halting phage propagation by terminating the infected host. The mechanisms by which Lamassu effectors are activated when needed and otherwise suppressed are unknown. Here we present structures of a Lamassu defense system from Salmonella enterica. We show that an oligomerization domain of the nuclease effector subunit, LmuA, is sequestered by two tightly folded SMC-like LmuB protomers and LmuC. Upon activation, liberated LmuA assembles into homotetramers, in which two of four nuclease domains are brought into proximity to create an active site capable of cleaving DNA. We propose that tetramer formation is likely a one-way switch that establishes a threshold to limit potential spontaneous activation and cell death. Our findings reveal a mechanism of cellular defense, involving liberation and oligomerization of immune effectors, and shed light on how Lamassu systems balance potent immune responses with self-preservation.

Summary: Lamassu is a diverse family of defence systems that protect bacteria, including pandemic strains of Vibrio cholerae , against phage infection. They target essential cellular processes, aborting infection and preventing phage propagation by terminating the infected host. The mechanisms by which Lamassu efectors are activated when needed and otherwise suppressed are unknown. Here, we present structures of a Lamassu defence system from Salmonella enterica . We show that an oligomerization domain of the nuclease efector, LmuA, is sequestered by two tightly-folded SMC-like LmuB protomers and LmuC. Upon activation, liberated LmuA proteins assemble into a cyclic homo-tetramer, in which two of four nuclease domains are brought into proximity to create an active site capable of cleaving DNA. We propose tetramer formation is likely a one-way switch that establishes a threshold to limit potential spontaneous activation and cell death. Our findings reveal a mechanism of cellular defence, involving liberation and oligomerization of immune efectors, and shed light on how Lamassu systems balance potent immune responses with self-preservation.

In bacteria, genes conferring antibiotic resistance are mostly carried on conjugative plasmids, mobile genetic elements that spread horizontally between bacterial hosts. Bacteria carry defence systems that defend them against genetic parasites, but how effective these are against plasmid conjugation is poorly understood. Here, we study to what extent restriction–modification (RM) systems—by far the most prevalent bacterial defence systems—act as a barrier against plasmids. Using 10 different RM systems and 13 natural plasmids conferring antibiotic resistance in Escherichia coli, we uncovered variation in defence efficiency ranging from none to 10⁵-fold protection. Further analysis revealed genetic features of plasmids that explain the observed variation in defence levels. First, the number of RM recognition sites present on the plasmids generally correlates with defence levels, with higher numbers of sites being associated with stronger defence. Second, some plasmids encode methylases that protect against restriction activity. Finally, we show that a high number of plasmids in our collection encode anti-restriction genes that provide protection against several types of RM systems. Overall, our results show that it is common for plasmids to encode anti-RM strategies, and that, as a consequence, RM systems form only a weak barrier for plasmid transfer by conjugation.

In bacteria, genes conferring antibiotic resistance are mostly carried on conjugative plasmids, mobile genetic elements which spread horizontally between bacterial hosts. Bacteria carry defence systems which defend them against genetic parasites, but how effective these are against plasmid conjugation is poorly understood. Here, we study to what extent Restriction-Modification (RM) systems – by far the most prevalent bacterial defence systems - act as a barrier against plasmids. Using 10 different RM systems and 13 natural plasmids conferring antibiotic resistance in Escherichia coli , we uncovered variation in defence efficiency ranging from none to 10 ⁵ -fold protection. Further analysis revealed genetic features of plasmids that explain the observed variation in defence levels. First, the number of RM recognition sites present on the plasmids generally correlates with defence levels, with higher numbers of sites being associated with stronger defence. Secondly, some plasmids encode methylases that protect against restriction activity. Finally, we show that a high number of plasmids in our collection encode anti-restriction genes that provide protection against several types of RM systems. Overall, our results show that it is common for plasmids to encode anti-RM strategies, and that, as a consequence, RM systems form only a weak barrier for plasmid transfer by conjugation.

Cleavage of bacteriophage DNA by the Type III restriction-modification enzymes requires long-range interaction between DNA sites. This is facilitated by one-dimensional diffusion (‘DNA sliding’) initiated by ATP hydrolysis catalyzed by a superfamily 2 helicase-like ATPase. Here we combined ultrafast twist measurements based on plasmonic DNA origami nano-rotors with stopped-flow fluorescence and gel-based assays to examine the role(s) of ATP hydrolysis. Our data show that the helicase-like domain has multiple roles. First, this domain stabilizes initial DNA interactions alongside the methyltransferase subunits. Second, it causes environmental changes in the flipped adenine base following hydrolysis of the first ATP. Finally, it remodels nucleoprotein interactions via constrained translocation of a ∼ 5 to 22-bp double stranded DNA loop. Initiation of DNA sliding requires 8–15 bp of DNA downstream of the motor, corresponding to the site of nuclease domain binding. Our data unify previous contradictory communication models for Type III enzymes.

Cleavage of bacteriophage DNA by the Type III Restriction-Modification enzymes requires long-range interaction between DNA sites. This is facilitated by 1-dimensional diffusion (“DNA sliding”) initiated by ATP hydrolysis catalysed by a superfamily 2 helicase-like ATPase. Here we combined ultrafast twist measurements based on plasmonic DNA origami nano-rotors with stopped-flow fluorescence and gel-based assays to examine the role(s) for ATP hydrolysis. Our data shows that the helicase-like domain plays multiple roles. Firstly, stabilising initial DNA-interactions alongside the methyltransferase subunits. Secondly, causing environmental changes in the flipped adenine base following hydrolysis of the first ATP. And thirdly, remodelling nucleoprotein interactions via constrained translocation of a ~5-20 bp dsDNA loop. Initiation of DNA sliding requires 10-15 bp DNA downstream of the motor, corresponding to the site of nuclease domain binding. Our data unifies previous contradictory communication models for Type III enzymes.