Structural basis of Retron-Eco8-mediated antiphage defense.

Area of Science:

• Bacteriology
• Molecular Biology
• Structural Biology

Background:

• Retrons are emerging bacterial defense mechanisms utilizing reverse transcriptase (RT), noncoding RNA, and effector proteins against phage predation.
• Understanding the precise molecular mechanisms of retron systems is crucial for deciphering bacterial immunity strategies.

Purpose of the Study:

• To elucidate the molecular mechanism and structural basis of the Retron-Eco8 system in bacterial defense.
• To investigate the role of the retron holocomplex and associated proteins in antiphage activity.

Main Methods:

• Biochemical assays to determine DNA cleavage activity of the Retron-Eco8 holocomplex.
• Cryo-electron microscopy (cryo-EM) to resolve the supramolecular structure of the defense complex.
• Phylogenetic analysis of single-stranded DNA-binding (SSB) proteins to understand resistance specificity.

Main Results:

• The Retron-Eco8 holocomplex, not just the effector, possesses double-stranded DNA cleavage activity, inducing abortive infection.
• Cryo-EM revealed a complex structure of four RT subunits, four multicopy single-stranded DNA molecules, and four OLD nucleases.
• Activation by various SSB proteins correlates with specific phage resistance, as shown by phylogenetic analysis.

Conclusions:

• The Retron-Eco8 complex functions as a cohesive unit for bacterial antiphage defense.
• Structural and mechanistic insights into retron systems advance our understanding of bacterial adaptive immunity and phage-host interactions.