Architecture and infection-sensing mechanism of the bacterial PARIS defense system.

Bacteria and the viruses that infect them (bacteriophages or phages) are engaged in an evolutionary arms race that has resulted in the development of hundreds of bacterial defense systems and myriad phage-encoded counterdefenses 1-5 . While the mechanisms of many bacterial defense systems are known 1 , how these systems avoid toxicity outside infection yet activate quickly upon sensing phage infection is less well understood. Here, we show that the bacterial P hage A nti- R estriction- I nduced S ystem (PARIS) operates as a toxin-antitoxin system, in which the antitoxin AriA sequesters and inactivates the toxin AriB until triggered by the T7 phage counterdefense protein Ocr. Using cryoelectron microscopy (cryoEM), we show that AriA is structurally similar to dimeric SMC-family ATPases but assembles into a distinctive homohexameric complex through two distinct oligomerization interfaces. In the absence of infection, the AriA hexamer binds up to three monomers of AriB, maintaining them in an inactive state. Ocr binding to the AriA-AriB complex triggers rearrangement of the AriA hexamer, releasing AriB and allowing it to dimerize and activate. AriB is a toprim/OLD-family nuclease whose activation arrests cell growth and inhibits phage propagation by globally inhibiting protein translation. Collectively, our findings reveal the intricate molecular mechanisms of a bacterial defense system that evolved in response to a phage counterdefense protein, and highlight how an SMC-family ATPase has been adapted as a bacterial infection sensor.