Disassembly activates Retron-Septu for antiphage defense.

Retrons are antiphage defense systems that produce multicopy single-stranded DNA (msDNA) and hold promise for genome engineering. However, the mechanisms of defense remain unclear. The Retron-Septu system integrates retron and Septu antiphage defenses. Cryo-electron microscopy structures reveal asymmetric nucleoprotein complexes comprising a reverse transcriptase, msDNA (a hybrid of msdDNA and msrRNA), and two PtuAB copies. msdDNA and msrRNA are essential for assembling this complex, with msrRNA adopting a conserved lariat-like structure that regulates reverse transcription. Notably, the assembled Retron-Septu complex is inactive, with msdDNA occupying the PtuA DNA binding site. Activation occurs upon disassembly, releasing PtuAB, which degrades single-stranded DNA to restrict phage replication. This "arrest-and-release" mechanism underscores the dynamic regulatory roles of msDNA, advancing our understanding of antiphage defense strategies.