Structural basis of Retron-Eco8-mediated anti-phage defense

Retrons represent a novel class of bacterial defense systems that employ reverse transcriptase (RT), non-coding RNA, and effector proteins to counteract phage infections. In this study, we elucidate the molecular mechanism of a retron system, Retron-Eco8. Biochemical experiments reveal that the Retron-Eco8 holocomplex, rather than the effector alone, exhibits double-stranded DNA cleavage activity in an ATP-independent manner, triggering abortive infection and effectively halting phage propagation. Cryo-electron microscopy (cryo-EM) analysis reveals a supramolecular assembly comprising four RT subunits, four msDNA (multicopy single-stranded DNA) molecules, and four OLD (overcoming lysogenization defect) nucleases-a configuration critical for anti-phage defense. Structural comparisons between apo and ATP-Mg2+-bound states demonstrate a local conformational change. Notably, we identify the phage SSB (single-stranded DNA-binding) protein as an activator of Retron-Eco8, and phylogenetic analysis of SSB proteins further elucidates the phage resistance specificity. Collectively, our findings delineate the structural architecture of the Retron-Eco8 defense complex and provide mechanistic insights into retron-mediated bacterial immunity.