Antiviral defence is a conserved function of diverse DNA glycosylases

Bacteria are frequently attacked by viruses, known as phages, and rely on diverse defence systems like restriction endonucleases and CRISPR-Cas to survive. While phages can evade these defences by covalently modifying their DNA, these non-canonical nucleobases create a strong selective pressure for host proteins that can recognize and exploit them. Here, using a structure-guided discovery approach, we identify widespread families of DNA glycosylases that protect bacteria against phages that incorporate modified guanine bases into their DNA. Despite high sequence variation, these enzymes share a conserved glycosylase fold and occur across bacterial lineages. We also uncover a distinct glycosylase superfamily that defends against phages with thymidine modifications, showing that glycosylases have repeatedly evolved as antiviral defences. Together, these findings reveal DNA glycosylases as versatile effectors of bacterial immunity and underscore structure-guided discovery as a powerful strategy for uncovering hidden layers of antiviral defence.