Susie Grigson (@susiegriggo.bsky.social)

Discovery of phage defense systems through component modularity networks Phage defense systems in bacteria exhibit high degrees of modularity, with sensing, signal transmission, and effector enzymes frequently being exchanged among phage defense gene clusters. In this study, we capitalized on this modularity to discover phage defense systems by searching for defense-associated modules in new gene contexts. This approach revealed a large and interconnected network of modular components distributed across diverse gene clusters. From over 500 candidate defense systems, we selected nine for experimental testing and validated three: Dionysus, a TerB-encoding system that disrupts early phage infection vesicle formation by Jumbo phages; Ophion, a Radical SAM-containing system that prevents the formation of the Jumbo phage nucleus; and Ambrosia, a tightly regulated RM-like system. Collectively, we demonstrate that leveraging the modular architecture of phage defense systems is an effective approach to their discovery. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, 101003229 Netherlands Organisation for Scientific Research, VI.C.192.027, OCENW.XS23.1.006 Koningin Wilhelmina Fonds, 15602 https://www.biorxiv.org/content/10.1101/2025.09.30.679545v1