A phage protein screen identifies triggers of the bacterial innate immune system

Bacteria have evolved sophisticated antiphage systems that halt phage replication upon detecting specific phage triggers. Identifying phage triggers is crucial to our understanding of immune signalling; however, they are challenging to predict. Here we used a plasmid library that expressed over 400 phage protein-coding genes from 6 phages to identify triggers of known and undiscovered antiphage systems. We transformed our library into 39 diverse strains of E. coli. Each strain natively harbours a different suite of antiphage systems whose activation typically inhibits growth. By tracking plasmids that were selectively depleted, we identified over 100 candidate phage trigger-E. coli pairs. Two phage proteins were further investigated, revealing that T7 gp17 and additional tail fibre proteins activated the undescribed antiphage system PD-T2-1 and identifying that λ gpE major capsid protein activated the antiphage system Avs8. These experiments provide a unique dataset for the continued definition of the molecular mechanisms underlying the bacterial immune system.