Structural insights into the assembly and function of Retron Ec78 PtuAB

The Retron system is a crucial prokaryotic anti-phage defense mechanism, composed of a noncoding RNA, reverse transcriptase, and effector proteins, which synthesizes multicopy single-stranded DNA to trigger immune responses. While Retron Ec86's anti-phage mechanism has been elucidated, other Retron types remain poorly understood. Here, we investigate Retron Ec78, a Type I-A system encoding effector proteins PtuA (ATPase) and PtuB (HNH nuclease). Structural analysis reveals that PtuA hexameric complex putatively bound to PtuB, with ATP playing a key role in hexamer assembly. Beyond its known role in abortive infection via tRNA degradation, we demonstrate that the PtuAB complex also exhibits DNA-cleaving activity. These findings provide novel insights into the assembly and dual functionality of the Retron Ec78 system, advancing our understanding of prokaryotic immune mechanisms against phage infection.