The DdmDE defense system eradicates plasmids by target-centered bidirectional ssDNA loop extrusion and site-specific cleavage

Research Hypothesis
The study investigates the molecular mechanism by which the DdmDE defense system in Vibrio cholerae identifies and eradicates invasive plasmids. The primary hypothesis is that the catalytically inert prokaryotic Argonaute (pAgo) DdmE serves as a force-responsive surveillance factor that recruits the multifunctional helicase-nuclease DdmD to specific DNA targets to initiate plasmid clearance.
Data Description and Gathering
The provided dataset contains raw and processed results obtained through a combination of single-molecule and biochemical techniques: Single-molecule Optical Tweezers & Fluorescence (C-Trap): Data gathered using a dual-trap system integrated with confocal microscopy to monitor the real-time binding, recruitment, and DNA processing activities of Cy3-labeled DdmE and eGFP-labeled DdmD/RPA on tethered λ DNA or engineered bubbled/gapped DNA constructs. Photobleaching Analysis: Step-wise fluorescence intensity data used to determine the stoichiometry of DdmE and DdmD at the target site. Biochemical Assays: Denaturing polyacrylamide gel electrophoresis (PAGE) data showing the cleavage of ssDNA substrates by DdmD. Next-Generation Sequencing (NGS): Sequencing data of DdmD cleavage products used to map preferred endonucleolytic cleavage sites on ssDNA. In vivo Assays: Bacterial growth and colony formation data from plasmid interference assays to validate the functional relevance of DdmD mutations.
Notable Findings
Force-Dependent Targeting: DdmE binding to dsDNA is facilitated by mechanical tension that induces transient DNA bubbles. Kinetic Discrimination: DdmE distinguishes on-targets from off-targets primarily through significantly different dissociation rates. Dimeric Recruitment: A stable, target-bound DdmE complex efficiently recruits DdmD as a dimer. Bidirectional Loop Extrusion: Once activated by ATP, DdmD drives bidirectional dsDNA unwinding and ssDNA loop extrusion while anchored at the target site. Arch Domain Engagement: The Arch domain of DdmD is essential for entrapping the excluding DNA strand during the extrusion process. Site-Specific Cleavage: DdmD acts as a slow endonuclease with a strong preference for cleavage upstream of guanine (5’-G) residues on ssDNA.
Data Interpretation and Use
The data supports a multi-step cooperative model for plasmid eradication. Interpretation: DdmE acts as a force-responsive targeting module that samples DNA for targets; once bound, it acts as a scaffold for DdmD. DdmD then executes a unique bidirectional unwinding/looping mechanism to expose ssDNA, which is subsequently degraded by free DdmD proteins in a sequence-specific manner. Usage: These datasets can be used to model the biophysics of pAgo-guided DNA interference, study motor protein-mediated DNA loop extrusion, or assist in the development of pAgo-based molecular tools for DNA targeting.