Molecular mechanism of plasmid elimination by the DdmDE defense system

Seventh pandemicVibrio choleraestrains contain two hallmark pathogenicity islands that encode the DNA defense modules DdmABC and DdmDE. Here we use cryo-EM to reveal the mechanistic basis for plasmid defense by DdmDE. A cryo-EM structure of the DdmD helicase-nuclease reveals that it adopts an auto-inhibited dimeric architecture. The prokaryotic Argonaute protein DdmE uses a DNA guide to target plasmid DNA. A structure of the DdmDE complex, validated byin vivomutational studies, shows that DNA binding by DdmE triggers disassembly of the DdmD dimer and loading of monomeric DdmD onto the non-target DNA strand. Finally,in vitrostudies reveal that DdmD translocates in the 5’ to 3’ direction, while partially degrading the plasmid DNA. These findings provide critical insights into the architecture and mechanism of DdmDE systems in plasmid elimination.