A phage parasite deploys a nicking nuclease effector to inhibit viral host replication.

Parasites of viruses, often referred to as satellites, are found in all domains of life and have been co-opted for host defense across diverse virus-host systems multiple independent times. This study describes the mechanism by which such an element prevents a bacterial virus (a 'phage') from otherwise infecting Vibrio cholera and related bacteria. The study is of broad interest to investigators with interests in phage-host interactions and microbial genetics.