DNA-measuring Wadjet SMC ATPases restrict smaller circular plasmids by DNA cleavage.

Abstract

Structural maintenance of chromosomes (SMC) complexes fold DNA by loop extrusion to support chromosome segregation, genome maintenance, and gene expression. Wadjet systems (JetABCD/MksBEFG/EptABCD) are derivative SMC complexes with proposed roles in bacterial immunity against selfish DNA elements. Here, we show that JetABCD systems restrict extrachromosomal circular DNA with an upper size limit of about 100 kb, while a linear plasmid evades restriction. Recombinant preparations of a JetABCD complex cleave circular DNA regardless of its helical topology but not linear DNA; cleavage occurs at random positions and depends on ATP as well as the SMC ATPase. We solve a structure of the JetABCD core by cryo-EM revealing an alternative dimer-of-dimers configuration. The two SMC DNA motor units face in opposite orientations--rather than the same as observed with MukBEF--possibly representing a restriction-specific state of JetABCD. We hypothesize that JetABCD is a DNA-shape-specific endonuclease and present a model for the activation of DNA cleavage exclusively when extrusion of an entire plasmid has been completed by a single JetABCD complex. The self-stalling of two opposing DNA motor units may serve as signal to trigger DNA cleavage. Complete extrusion by a single complex cannot be achieved on the much larger chromosome, explaining how self-DNA may evade cleavage.