Anti-viral defense by an ADP-ribosyltransferase that targets mRNA to block translation

Host-pathogen conflicts are crucibles of molecular innovation. Selection for immunity to pathogens has driven the evolution of sophisticated immunity mechanisms throughout biology, including in bacteria that must evade their viral predators known as bacteriophages. Here, we characterize a widely distributed anti-phage defense system, CmdTAC, that provides robust defense against infection by the T-even family of phages. Our results support a model in which CmdC detects infection by sensing viral capsid proteins, ultimately leading to the activation of a toxic ADP-ribosyltransferase effector protein, CmdT. We show that newly synthesized capsid protein triggers dissociation of the chaperone CmdC from the CmdTAC complex, leading to destabilization and degradation of the antitoxin CmdA, with consequent liberation of the CmdT ADP-ribosyltransferase. Strikingly, CmdT does not target a protein, DNA, or structured RNA, the known targets of other ADP-ribosyltransferases. Instead, CmdT modifies the N6 position of adenine in GA dinucleotides within single-stranded RNAs leading to arrest of mRNA translation and the inhibition of viral replication. Our work reveals a new mechanism of anti-viral defense and a previously unknown but broadly distributed class of ADP-ribosyltransferases that target mRNA.