Structural mechanism of the Retron-Eco7 anti-phage defense system

Retrons are prokaryotic genetic elements involved in anti-phage defense and consist of a non-coding RNA, a reverse transcriptase (RT), and various effector proteins. Retron-Eco7 (previously known as Retron-Ec78) from Escherichia coli encodes two effector proteins (the PtuA ATPase and the PtuB nuclease) and degrades the host tRNA^Tyr upon phage infection, thereby protecting host cells against invading phages. However, its defense mechanism remains elusive. Here, we report the cryo-electron microscopy (cryo-EM) structures of the Retron-Eco7 complex, comprising the RT, multicopy single-stranded DNA (msDNA), PtuA, and PtuB. The Retron-Eco7 structures reveal that the RT-msDNA complex associates with two PtuA-PtuB complexes, potentially inhibiting their nuclease activity and suppressing bacterial growth arrest prior to phage infection. Furthermore, the phage-encoded D15 nuclease acts as a trigger for the Retron-Eco7 system and cleaves the msDNA bound to the complex, facilitating the dissociation of PtuA-PtuB from RT-msDNA. Our data indicate that msDNA cleavage by D15 is the initial step required for the specific cleavage of the host tRNA^Tyr by the PtuA-PtuB nuclease, which leads to abortive infection. Overall, this study provides mechanistic insights into the Retron-Eco7 system and highlights the diversity of prokaryotic anti-phage defense mechanisms.