Disassembly activates Retron-Septu for antiphage defense.

Area of Science:

• Microbiology
• Molecular Biology
• Structural Biology

Background:

• Retrons are bacterial defense systems producing multicopy single-stranded DNA (msDNA).
• The precise mechanisms of retron-mediated antiphage defense are not fully understood.
• The Retron-Septu system represents a unique combination of retron and Septu defense mechanisms.

Purpose of the Study:

• To elucidate the structural basis and mechanism of the Retron-Septu antiphage defense system.
• To understand the role of msDNA and its components in complex assembly and regulation.
• To investigate the activation and function of the Retron-Septu complex in phage defense.

Main Methods:

• Cryo-electron microscopy (Cryo-EM) to determine the structure of the Retron-Septu complex.
• Biochemical analysis to assess the roles of msdDNA and msrRNA in complex formation.
• Functional assays to study the activity of the assembled complex and its components.

Main Results:

• Cryo-EM revealed asymmetric nucleoprotein complexes containing reverse transcriptase (RT), msDNA (msdDNA and msrRNA), and PtuAB.
• msdDNA and msrRNA are crucial for complex assembly, with msrRNA forming a regulatory lariat-like structure.
• The assembled Retron-Septu complex is inactive, with msdDNA blocking the PtuA DNA-binding site.
• Complex disassembly releases PtuAB, which degrades single-stranded DNA to inhibit phage replication.

Conclusions:

• The Retron-Septu system employs an "arrest-and-release" mechanism for antiphage defense.
• msDNA plays a dynamic regulatory role in controlling retron activity.
• This study advances the understanding of bacterial defense strategies and msDNA function.