Viruses encode tRNA and anti-retron to evade bacterial immunity

Abstract

Retrons are bacterial genetic retroelements that encode reverse transcriptase capable of producing multicopy single-stranded DNA (msDNA) and function as antiphage defense systems. Phages employ several strategies to counter the host defense systems, but no mechanisms for evading retrons are known. Here, we show that tRNATyr and Rad (retron anti-defense) of T5 phage family inhibit the defense activity of retron 78 and a broad range of retrons, respectively. The effector protein of retron 78, ptuAB, specifically degraded tRNATyr leading abortive infection, but phage countervailed this defense by supplying tRNATyr. Rad inhibited retron function by degrading noncoding RNA, the precursor of msDNA. In summary, we demonstrated that viruses encode at least two independent strategies for overcoming bacterial defense systems: anti-defense, such as Rad, and defense canceler, like tRNA.