The MksG nuclease is the executing part of the bacterial plasmid defense system MksBEFG

Cells are continuously facing the risk of taking up foreign DNA that can compromise genomic integrity. Therefore, bacteria are in a constant arms race with mobile genetic elements such as phages, transposons and plasmids. They have developed several active strategies against invading DNA molecules that can be seen as a bacterial 'innate immune system'. Here, we investigated the molecular arrangement of the Corynebacterium glutamicum MksBEFG complex, which is homologous to the MukBEF condensin system. We show here that MksG is a nuclease that degrades plasmid DNA. The crystal structure of MksG revealed a dimeric assembly through its C-terminal domain that is homologous to the TOPRIM domain of the topoisomerase II family of enzymes and contains the corresponding ion binding site essential for DNA cleavage in topoisomerases. The MksBEF subunits exhibit an ATPase cycle in vitro and we reason that this reaction cycle, in combination with the nuclease activity provided by MksG, allows for processive degradation of invading plasmids. Super-resolution localization microscopy revealed that the Mks system is spatially regulated via the polar scaffold protein DivIVA. Introduction of plasmids results in an increase in DNA bound MksG, indicating an activation of the system in vivo.

Automated summary

Cells face the constant risk of foreign DNA invasion, and bacteria have developed strategies similar to an innate immune system to defend against it. This study investigates the MksBEFG complex in Corynebacterium glutamicum, which degrades plasmid DNA. The structure of MksG reveals that it acts as a nuclease and has a binding site essential for DNA cleavage. The Mks system is regulated through the polar scaffold protein DivIVA and is activated when plasmids are introduced.