Phages reconstitute NAD⁺ to counter bacterial immunity

Bacteria defend against phage infection through a variety of antiphage defence systems¹. Many defence systems were recently shown to deplete cellular nicotinamide adenine dinucleotide (NAD⁺) in response to infection, by cleaving NAD⁺ into ADP-ribose (ADPR) and nicotinamide^2-7. It was demonstrated that NAD⁺ depletion during infection deprives the phage of this essential molecule and impedes phage replication. Here we show that a substantial fraction of phages possess enzymatic pathways allowing reconstitution of NAD⁺ from its degradation products in infected cells. We describe NAD⁺ reconstitution pathway 1 (NARP1), a two-step pathway in which one enzyme phosphorylates ADPR to generate ADPR pyrophosphate (ADPR-PP), and the second enzyme conjugates ADPR-PP and nicotinamide to generate NAD⁺. Phages encoding NARP1 can overcome a diverse set of defence systems, including Thoeris, DSR1, DSR2, SIR2-HerA and SEFIR, all of which deplete NAD⁺ as part of their defensive mechanism. Phylogenetic analyses show that NARP1 is primarily encoded on phage genomes, suggesting a phage-specific function in countering bacterial defences. A second pathway, NARP2, allows phages to overcome bacterial defences by building NAD⁺ using metabolites different from ADPR-PP. Our findings reveal a unique immune evasion strategy in which viruses rebuild molecules depleted by defence systems, thus overcoming host immunity.