Hachiman is a genome integrity sensor

Hachiman is a broad-spectrum antiphage defense system of unknown function. We show here that Hachiman comprises a heterodimeric nuclease-helicase complex, HamAB. HamA, previously a protein of unknown function, is the effector nuclease. HamB is the sensor helicase. HamB constrains HamA activity during surveillance of intact dsDNA. When the HamAB complex detects DNA damage, HamB helicase activity liberates HamA, unleashing nuclease activity. Hachiman activation degrades all DNA in the cell, creating ‘phantom’ cells devoid of both phage and host DNA. We demonstrate Hachiman activation in the absence of phage by treatment with DNA-damaging agents, suggesting that Hachiman responds to aberrant DNA states. Phylogenetic similarities between the Hachiman helicase and eukaryotic enzymes suggest this bacterial immune system has been repurposed for diverse functions across all domains of life. ### Competing Interest Statement J.A.D. is a co-founder of Caribou Biosciences, Editas Medicine, Intellia Therapeutics, Mammoth Biosciences and Scribe Therapeutics, and a director of Altos, Johnson & Johnson and Tempus. J.A.D. is a scientific advisor to Caribou Biosciences, Intellia Therapeutics, Mammoth Biosci-ences, Inari, Scribe Therapeutics, Felix Biosciences and Algen. J.A.D. also serves as Chief Science Advisor to Sixth Street and a Scientific Advi-sory Board member at The Column Group. J.A.D. conducts academic research projects sponsored by Roche and Apple Tree Partners. J.P. has an equity interest in Linnaeus Bioscience Incorporated and receives income. The terms of this arrangement have been reviewed and approved by the University of California, San Diego, in accordance with its conflict-of-interest policies.