CapRel is a fused toxin–antitoxin system that is active against diverse phages when expressed in *Escherichia coli*. CapRel belongs to the family of toxSAS toxin–antitoxin systems. CapRel is an Abortive infection system which is found in Cyanobacteria, Actinobacteria, and Proteobacteria, Spirochetes, Bacteroidetes, and Firmicutes, as well as in some temperate phages.
CapRel is a fused toxin-antitoxin system that is active against diverse phages when expressed in *Escherichia coli* :ref{doi=10.1038/s41586-022-05444-z}. CapRel belongs to the family of toxSAS toxin-antitoxin systems. CapRel is an Abortive infection system which is found in Cyanobacteria, Actinobacteria, and Proteobacteria, Spirochetes, Bacteroidetes, and Firmicutes, as well as in some temperate phages.
## Molecular mechanism
The CapRel system of Salmonella temperate phage SJ46 is normally found in a closed conformation, which is thought to maintain CapRel in an auto-inhibited state. However during phage SECPhi27 infection, binding of the major phage capsid protein (Gp57) to CapRel releases it from is inhibited state, allowing pyrophosphorylation of tRNAs by the toxin domain and resulting in translation inhibition. Other phage capsid proteins can be recognized by CapRel, as observed during infection by phage Bas8.
The CapRel system of Salmonella temperate phage SJ46 is normally found in a closed conformation, which is thought to maintain CapRel in an auto-inhibited state. However during phage SECPhi27 infection, binding of the major phage capsid protein (Gp57) to CapRel releases it from is inhibited state, allowing pyrophosphorylation of tRNAs by the toxin domain and resulting in translation inhibition :ref{doi=10.1038/s41586-022-05444-z}. Other phage capsid proteins can be recognized by CapRel, as observed during infection by phage Bas8.
Different CapRel homologues confer defense against different phages, suggesting variable phage specificity of CapRel system which seems to be mediated by the C-terminal region of CapRel.
