Retrons are bacterial genetic elements comprised of a reverse transcriptase (RT) and a non-coding RNA (ncRNA). The RT uses the ncRNA as template, generating a chimeric RNA/DNA molecule in which the RNA and DNA components are covalently linked. Although retrons were discovered three decades ago, their function remained unknown. We report that retrons function as anti-phage defense systems. The defensive unit is composed of three components: the RT, the ncRNA, and an effector protein. We examined multiple retron systems and show that they confer defense against a broad range of phages via abortive infection. Focusing on retron Ec48, we show evidence that it “guards” RecBCD, a complex with central anti-phage functions in bacteria. Inhibition of RecBCD by phage proteins activates the retron, leading to abortive infection and cell death. Thus, the Ec48 retron forms a second line of defense that is triggered if the first lines of defense have collapsed.

Retrons是由逆转录酶（RT）和非编码RNA（ncRNA）组成的细菌遗传元件。RT使用ncRNA作为模板，生成嵌合RNA / DNA分子，其中RNA和DNA组分共价连接。尽管逆回溯是在三十年前发现的，但其功能仍然未知。我们报告说，retroterns作为抗噬菌体防御系统。防御单元由三部分组成：RT，ncRNA和效应蛋白。我们检查了多个逆转录系统，并显示它们通过流产感染赋予了对广泛噬菌体的防御能力。着重于Ec48的逆转录，我们显示出它可以“保护” RecBCD（一种在细菌中具有中央抗噬菌体功能的复合物）。噬菌体蛋白对RecBCD的抑制会激活逆转录，从而导致流产感染和细胞死亡。从而，