Coordinated synthesis of double-stranded DNA by a dual reverse transcriptase immune system

Recent studies have revealed that defense-associated reverse transcriptase (DRT) systems mediate an-tiviral immunity through distinct modes of cDNA synthesis. Class I DRTs catalyze untemplated DNA synthesis with random or nucleotide-biased sequences, whereas Class II DRTs polymerize noncoding RNA-templated products, including concatemeric repeats and homopolymeric cDNA. However, how these distinct modes of cDNA synthesis are employed to drive antiviral defense remains poorly under-stood. Here, we report an unprecedented mechanism of DRT3 immunity, in which RT enzymes from both Class I and Class II coordinate their diverse activities to produce self-complementary double-stranded DNA (dsDNA). Remarkably, whereas the DRT3a enzyme relies on a 5'-ACACAC-3' RNA template to synthesize long poly-(dTdG) repeats, DRT3b synthesizes precise poly-(dCdA) repeats without any nucle-ic acid template at all. Cryo-electron microscopy structures reveal that DRT3b assembles into a hexameric complex and employs active site-adjacent residues to function as deoxyadenosine and deoxycytidine gates that enforce alternating addition to produce dinucleotide repeats, representing a unique example of amino acid-templated DNA polymerization. Strikingly, DRT3 immune systems are toxic in a genetic background lacking E. coli RecBCD, implicating host recombination machinery in limiting DRT3-mediated dsDNA levels. Consistent with this model, we discovered that the phage-encoded RecBCD inhibitor, Gam, po-tently triggers DRT3-mediated abortive infection. Collectively, our findings reveal how two polymerases with distinct templating strategies cooperate to generate complementary DNA and drive antiviral defense.