Shield co-opts an RmuC domain to mediate phage defence across Pseudomonas species.

Competitive bacteria-bacteriophage interactions have resulted in the evolution of a plethora of bacterial defense systems preventing phage propagation. In recent years, computational and bioinformatic approaches have underpinned the discovery of numerous novel bacterial defense systems. Anti-phage systems are frequently encoded together in genomic loci termed defense islands. Here we report the identification and characterisation of a novel anti-phage system, which we have termed Shield, that forms part of the Pseudomonas defensive arsenal. The Shield system comprises a membrane-bound protein, ShdA, harboring an RmuC domain. Heterologous production of ShdA alone is sufficient to mediate bacterial immunity against a panel of phages. We show that ShdA homologues can degrade phage DNA in vitro and, when expressed in a heterologous host, can alter the organisation of chromosomal DNA to a nucleoid structure. Further analysis reveals that Shield can be divided into four subtypes, three of which contain additional components that in some cases can modulate the activity of ShdA and/or provide additional lines of phage defence. Collectively, our results identify a new player within the Pseudomonas bacterial immunity arsenal that displays a novel mechanism of protection, and reveals a surprising role of RmuC domains in phage defence. SIGNIFICANCEThe evolutionary pressure exerted by bacteriophages has driven bacteria to acquire numerous defense systems. Recent studies have highlighted the extraordinary diversity of these systems, uncovering exciting links between bacterial and eukaryotic immunity. Here we describe a novel anti-phage system, named Shield, found within Pseudomonas species. We identify several Shield subtypes, all harboring the same core component, and describe its mode of action. The growing instance of multidrug-resistant bacterial infections urgently requires the development of alternative treatments. Phage therapy is a particularly pertinent approach to treat multi-drug resistant Pseudomonas aeruginosa strains causing severe lung infection in cystic fibrosis patients. A detailed understanding of bacterial immunity and phage counter-strategies is an essential step to underpin the rational design of phage therapy to fight disease.