We propose Hachiman activates in response to abberant DNA states. DNA ends load into the helicase, triggering ATP hydrolysis and release of HamA. HamA chews up DNA, potentially amplifying the immune response by creating more sites for HamAB loading. 11/14

In another cryo-EM structure, we visualized nuclease-dead HamAB binding intact DNA in an orthogonal mode. We imagine this 'sliding' or binding state enables the helicase to thread ssDNA lesions into the active site. 10/14

But why is Hachiman so effective against diverse phages? The trigger must be central to phage infection. We show that direct DNA damage induced by a drug activates Hachiman in the absence of phage! This key insight implies HamAB recognizes DNA damage as a viral signature. 9/14

Our collaborators Emily Armbruster and Joe Pogliano @ucsd showed Hachiman in action. When infected with phage, HamB completely shreds both phage and host DNA, leaving behind "phantom cells" devoid of genomes. Infected cells sacrifice themselves in service of the colony. 8/14

In another surprise, we discovered HamB is a DNA helicase! A set of structures shows DNA entering the HamB active site. We resolved 'ratcheting' states that lead to large domain movements upon activation. This motion may release the HamA nuclease. 7/14

The structure led us to identify an active site for the HamA DUF! HamA is a nuclease homologous to nucleases in Type IIS restriction modification systems. HamA insertions mediate binding to HamB. The HamAB complex cuts DNA in vitro. 6/14

To get at the mechanism, we turned to cryo-EM. HamA and HamB form a heterodimeric complex, with HamA contacting an N-terminal helical bundle in HamB. 5/14

We identified Hachiman loci in E. coli isolates. ECOR31 Hachiman had strong, broad-spectrum activity against diverse bacteriophages. 4/14

The second, HamB, is a superfamily 2 Ski2/Brr2-like helicase. The homology to RNA helicases intrigued us. In bioinformatic work led by @benadler.bsky.social, we contextualize HamB, showing general helicase enrichment in immune systems across domains of life. 3/14

@soreklab.bsky.social discovered a host of prokaryotic immune systems in a seminal 2018 work. One system - Hachiman - caught our eye. Named for a Japanese protector deity, Hachiman is composed of only two genes. The first, HamA, contains only a domain of unknown function (DUF1837). 2/14