A DNA nicking Class 1 OLD family nuclease mediates phage defense in <i>Vibrio cholerae</i> and is countered by a phage-encoded inhibitor

Bacteria are constantly threatened by their viral predators (phages), which has resulted in the development of defense systems for bacterial survival. One family of defense systems found widely across bacteria are OLD (for overcoming lysogenization defect) family nucleases. Despite recent discoveries regarding Class 2 and 4 OLD family nucleases and how phages overcome them, Class 1 OLD family nucleases remain underexplored, with only one antiphage Class 1 OLD family nuclease described to date. Here, we identify the first bacterially encoded Class 1 OLD family nuclease with antiphage activity: the Vibrio cholerae-encoded Vc OLD. We describe its disruption of genome replication of the lytic vibriophage ICP1. Furthermore, we examine its in vitro activity, identifying Vc OLD as a DNA nickase. Finally, we identify the first direct inhibitor of a Class 1 OLD family nuclease, the ICP1-encoded Oad1. Our research sheds new light on the role of Class 1 OLD family nucleases in phage defense and highlights the dynamic arms race between V. cholerae and its predatory phage ICP1.