SauUSI belongs to the type IV family of restriction endonucleases, enzymes that specialize in attacking DNA carrying chemical modifications rather than specific unmodified sequences. Its target is 5-methylcytosine, or 5mC, and its cousin 5-hydroxymethylcytosine, embedded within a loosely defined recognition motif. These modifications are the camouflage of choice for bacteriophages and plasmids seeking to slip past the classical restriction-modification and CRISPR-Cas defenses of their bacterial hosts. SauUSI turns that camouflage into a liability: any piece of DNA bearing the methyl tag is flagged as nonself and condemned. In S. aureus, the enzyme acts as a major barrier to horizontal gene transfer, shaping the acquisition of virulence factors, pathogenicity islands and antimicrobial resistance genes carried on methylated prophages and plasmids. Homologs are widespread across Gram-positive and Gram-negative bacteria and even some archaea, making the mechanism of broad evolutionary and clinical significance.

The enzyme’s architecture hints at its dual personality. Two SauUSI monomers dimerize through N-terminal phospholipase D-family nuclease domains, forming a single active site capable of cleaving both DNA strands sequentially, much like the well-studied type II enzyme BfiI. Each monomer also carries dual helicase-like ATPase domains and a C-terminal target recognition domain, or TRD, with an SRA fold that presumably flips the modified base into a recognition pocket. Earlier work established that SauUSI consumes ATP to move along DNA and cuts 5 to 20 base pairs from its targets, but the polarity of motion, the number of active motors and the trigger for nuclease activation all remained unresolved. The Bristol team therefore combined ensemble biochemistry, single-molecule manipulation and nanopore sequencing to watch the enzyme at work.

The first surprise came from triplex displacement assays, in which fluorescent triplex-forming oligonucleotides anchored at defined positions report the passage of an incoming motor. Using DNA methylated on one or both strands by site-specific methyltransferases, the researchers showed that SauUSI translocates unidirectionally, moving 3-prime to 5-prime along the methylated strand. On symmetrically methylated DNA, displacement occurred on both sides of the target, a clue that both strands can be engaged. Stopped-flow measurements with varying distances between the methylated site and the triplex yielded a translocation rate of roughly 180 base pairs per second at 25 degrees Celsius, with negligible dissociation during motion. Fitting the data revealed an unexpected detail: the motor does not start immediately adjacent to the methylated base but binds about 22 base pairs upstream, a geometry confirmed by an AlphaFold3 model of the DNA-bound dimer.

ATP-dependence experiments pinned the energetics to a single engine. As ATP concentrations fell, both translocation and ATP hydrolysis rates declined in parallel, giving a coupling ratio of approximately one ATP consumed per base pair traveled. That stoichiometry is the signature of one helicase-like subunit driving unidirectional movement, not two motors pulling in tandem. The picture that emerges is elegant: upon binding 5mC, one subunit’s ATPase initiates translocation while the motor physically pushes toward its own TRD, which cannot be bypassed and is displaced from the modified site. What happens next determines the fate of the DNA molecule.

Single-molecule experiments using dual optical traps and confocal microscopy, performed on a LUMICKS C-trap, exposed that fate in vivid detail. With a hemimethylated DNA tether held between beads and ATP added, the team observed unidirectional translocation in 46 percent of events coinciding with an increase in DNA tension, the hallmark of a growing loop. The displaced TRD, it turns out, can recapture the liberated methylated site in roughly half the events, anchoring the DNA while the motor keeps walking, swelling the loop. In the other half, the motor releases the site entirely and glides freely, sometimes for thousands of base pairs to a DNA end. A nuclease-dead mutant observed in magnetic tweezers, and a topoisomerase supercoiling assay that detects the twisted domains generated by looped translocation, independently confirmed the looping mechanism and its dependence on both the TRDs and the freed 5mC.

When both strands carry the methyl mark, the enzyme becomes even more formidable. Looping frequency on dimethylated DNA rose to 73 percent, and at least half of those loops involved bidirectional translocation, with one motor pulling each way from the anchored methylated site. Bidirectional loop translocation ran at 548 plus or minus 59 base pairs per second in the C-trap, slightly more than double the unidirectional rate, suggesting a degree of allosteric activation between the two ATPase subunits rather than completely independent motors. Force, applied across a range up to 20 piconewtons, did not slow the motors, consistent with the tiny one-base-pair steps they take. The authors also reconciled rate discrepancies between instruments by accounting for laser-induced heating in the optical traps, applying a measured Q10 temperature coefficient to the comparison.

Mapping where the enzyme cuts brought the mechanism full circle. Using ENDO-Pore, a high-throughput nanopore method that resolves double-strand breaks on single DNA molecules, the team showed that primary cleavage occurs stochastically throughout translocation, both near and far from the methylated site, generating mostly one-nucleotide 3-prime overhangs and blunt ends. Cleavage clustered within a 20-base-pair window about 22 base pairs upstream of the 5mC, exactly where a rebinding TRD would park the nuclease, and was strongly enriched at 5-prime-TA-3-prime and 5-prime-TNA-3-prime dinucleotides at distal loci. Crucially, a single dimer proved sufficient for both primary and secondary cleavage, with no evidence that converging motors or protein roadblocks are required for activation. Instead, anything that prolongs the motor’s dwell time, whether 5mC rebinding, motor congestion at high enzyme concentrations, or a dCas9 roadblock deliberately placed on the DNA, raises the probability of a cut. On DNA bearing two methylated sites, single molecules were even seen switching direction, patrolling back and forth between sites in a ping-pong scan that multiplies cleavage opportunities.

The overall strategy, which the authors call iterative translocation-cleavage, explains why methylated DNA is so efficiently fragmented even at low enzyme concentrations: repeated cycles of 5mC recognition, ATP-driven movement and stochastic cutting progressively dice the invader into small pieces, without any need for enzyme-enzyme communication and without releasing a diffusible nuclease that could harm the host. Nuclease activity is constrained in cis, tied to the DNA the enzyme is patrolling, so SauUSI functions as a first-line innate defense that neutralizes threats without suicide, in contrast to abortive systems such as Lamassu. Because N6-methyladenine and N4-methylcytosine, the modifications most common in bacteria, are not recognized, the enzyme spares typical bacterial genomes, though 5mC-modified S. aureus strains could never acquire a SauUSI-like system without self-destruction. The work illuminates how helicase-like motors, ubiquitous in bacterial defense systems, can be repurposed from duplex unwinding into destructive surveillance, and it may guide understanding of related motor-nuclease enzymes such as CglI and NgoAVII. Coupling a nuclease to a processive motor, the authors suggest, is an effective way to deliver multiple, distantly spaced, hard-to-repair breaks, and the resulting fragments might even feed CRISPR-Cas adaptation.

Subject of Research: Mechanism of the ATP-dependent type IV restriction enzyme SauUSI cleaving methylated DNA in Staphylococcus aureus

Article Title: Single Staphylococcus aureus SauUSI restriction enzyme motors fragment methylated DNA

Article References: Single Staphylococcus aureus SauUSI restriction enzyme motors fragment methylated DNA. (n.d.). https://doi.org/10.1038/s41589-026-02353-4

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02353-4

Keywords: SauUSI, Staphylococcus aureus, restriction enzyme, DNA methylation, 5-methylcytosine, helicase, DNA translocation, single-molecule biophysics, nuclease, bacterial immunity, horizontal gene transfer, antimicrobial resistance