Bacteria defend themselves from viral predation using diverse immune systems, many of which target foreign DNA for degradation¹. 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 that encodes an antiviral effector protein, Neo^4,5. It remains unknown whether similar mechanisms of defense are employed by other related DRT families. Focusing on DRT9, here we uncover an unprecedented mechanism of DNA homopolymer synthesis. Viral infection triggers polydeoxyadenylate (poly-dA) accumulation in the cell, driving abortive infection and population-level immunity. Cryo-EM structures reveal how a noncoding RNA serves as both a structural scaffold and reverse transcription template to direct hexameric complex assembly and poly-dA synthesis. Remarkably, biochemical and functional experiments identify tyrosine residues within the reverse transcriptase itself that likely prime DNA synthesis, leading to the formation of high-molecular weight protein-DNA covalent adducts. Synthesis of poly-dA by DRT9 in vivo is regulated by the competing activities of phage-encoded triggers and host-encoded silencers. 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.

细菌使用不同的免疫系统来保护自己免受病毒捕食，其中许多免疫系统以外源 DNA 为靶点进行降解 ¹。防御相关逆转录酶 （DRT） 系统通过利用 DNA 合成代替 ^2,3 为这种策略提供了一个有趣的对立面。我们和其他人最近表明，DRT2 系统使用 RNA 模板组装编码抗病毒效应蛋白 Neo^4,5 的从头基因。目前尚不清楚其他相关的 DRT 家族是否采用类似的防御机制。专注于 DRT9，我们在这里发现了一种前所未有的 DNA 均聚物合成机制。病毒感染会触发聚脱氧腺苷酸酯 （poly-dA） 在细胞中的积累，从而驱动流产感染和群体水平的免疫力。冷冻电镜结构揭示了非编码 RNA 如何作为结构支架和逆转录模板来指导六聚体复合物组装和 poly-dA 合成。值得注意的是，生化和功能实验确定了逆转录酶本身内的酪氨酸残基，这些残基可能引发 DNA 合成，导致形成高分子量蛋白质-DNA 共价加合物。DRT9 在体内合成 poly-dA 受噬菌体编码的触发器和宿主编码的沉默剂的竞争活性的调节。总的来说，我们的工作揭示了一种新的核酸驱动的防御系统，该系统扩展了细菌免疫的范式并拓宽了逆转录酶的已知功能。