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 cryo-EM structures of apo- and 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/ExK 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.