# Structural basis of Retron–Eco8-mediated antiphage defense

## TL;DR

This paper explains how the Retron–Eco8 system in bacteria uses a complex of proteins and DNA to defend against phage infections.

## Contribution

The study reveals the structural and functional mechanism of the Retron–Eco8 defense system, including its activation by SSB proteins.

## Key findings

- The Retron–Eco8 holocomplex cleaves DNA to stop phage propagation.
- Cryo-EM shows the complex includes four RT subunits and OLD nucleases.
- SSB proteins activate the system and determine phage resistance specificity.

## Abstract

Retrons represent a novel class of bacterial defense systems that employ reverse transcriptase (RT), noncoding 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, triggering abortive infection and therefore effectively halting phage propagation. Cryo-electron microscopy (cryo-EM) analysis reveals a supramolecular assembly comprising four RT subunits, four multicopy single-stranded DNA molecules, and four overcoming lysogenization defect (OLD) nucleases—a configuration critical for antiphage defense. Notably, we examine the activation of Retron–Eco8 by diverse single-stranded DNA-binding (SSB) proteins, and phylogenetic analysis of these SSB proteins 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.

Graphical Abstract