Bacterial Schlafen proteins mediate phage defence.

Human Schlafen proteins restrict viral replication by cleaving tRNA, thereby suppressing protein synthesis. Although the ribonuclease domain of Schlafen proteins is conserved across all domains of life, its function in prokaryotes has remained unclear. Here we demonstrate that prokaryotic Schlafen nucleases are widespread antiviral effectors that protect bacteria from bacteriophages and are fused to a diverse array of phage-sensing domains. We expressed seven Enterobacterales Schlafen systems in Escherichia coli, identifying two that confer defence against coliphages. We focused on a system where Schlafen nuclease is fused to a previously unknown immunoglobulin-like sensor domain and demonstrated that it recognizes tail assembly chaperones of T5-like phages. Upon activation, the Schlafen nuclease cleaves both E. coli and phage-encoded tRNAs and restricts T5 phage by reducing its burst size. Our findings redefine Schlafens as an ancient, mechanistically conserved family of immune effectors, revealing the deep evolutionary origin of tRNA-targeting antiviral immunity in humans.