Protein-primed DNA homopolymer synthesis by an antiviral reverse transcriptase.

Bacteria defend themselves from viral predation using diverse immune systems, many of which sense and target foreign DNA for degradation(1). Defense-associated reverse transcriptase (DRT) systems provide an intriguing counterpoint to this strategy by leveraging DNA synthesis instead(2,3). We and others recently showed that DRT2 systems use an RNA template to assemble a de novo gene, leading to expression of an antiviral effector protein, Neo(4,5). It remains unknown whether similar mechanisms of defense are employed by other DRT families. Focusing on DRT9, here we uncover an unprecedented mechanism of DNA homopolymer synthesis, in which viral infection triggers polydeoxyadenylate (poly-dA) accumulation in the cell to drive abortive infection and population-level immunity. Cryo-EM structures reveal how a conserved noncoding RNA serves as both a structural scaffold and reverse transcription template to direct hexameric complex assembly and RNA-templated poly-dA synthesis. Remarkably, biochemical and functional experiments identify conserved tyrosine residues within the reverse transcriptase itself that prime DNA synthesis, leading to the formation of high-molecular weight protein-DNA covalent adducts. Synthesis of poly-dA in vivo is regulated by the competing activities of phage-encoded triggers and host-encoded silencers of DRT9. Collectively, our work unveils a novel nucleic acid-driven defense system that expands the paradigm of bacterial immunity and broadens the known functions of reverse transcriptases.