Toxin-antitoxin systems (TAs) are abundant in bacterial chromosomes and can arrest growth under stress, but usually remain inactive. TAs have been increasingly implicated in halting the growth of infected bacteria from bacteriophages or foreign genetic elements^(1,2) to protect the population (abortive infection, Abi). The vast diversity and abundance of TAs and other Abi systems³ suggest they play an important immunity role, yet what allows them to sense attack remains largely enigmatic. …