Bacteria have evolved diverse mechanisms to fend off predation by bacteriophages. We previously identified the Dnd system, which uses DndABCDE to insert sulfur into the DNA backbone as a double-stranded phosphorothioate (PT) modification, and DndFGH, a restriction component. Here, we describe an unusual SspABCD–SspE PT system in Vibrio cyclitrophicus, Escherichia coli and Streptomyces yokosukanensis, which has distinct genetic organization, biochemical functions and phenotypic behaviour. SspABCD confers single-stranded and high-frequency PTs with SspB acting as a nickase and possibly introducing nicks to facilitate sulfur incorporation. Strikingly, SspABCD coupled with SspE provides protection against phages in unusual ways: (1) SspE senses sequence-specific PTs by virtue of its PT-stimulated NTPase activity to exert its anti-phage activity, and (2) SspE inhibits phage propagation by introducing nicking damage to impair phage DNA replication. These results not only expand our knowledge about the diversity and functions of DNA PT modification but also enhance our understanding of the known arsenal of defence systems.

细菌已经进化出多种机制来抵御噬菌体的捕食。我们之前确定了 Dnd 系统，该系统使用 DndABCDE 将硫作为双链硫代磷酸酯 (PT) 修饰插入 DNA 主链，以及 DndFGH，一种限制性成分。在这里，我们描述了一种不寻常的 SspABCD-SspE PT 系统，它存在于Vibrio cyclotrophicus、Escherichia coli和Streptomyces yokosukanensis 中。，具有独特的遗传组织、生化功能和表型行为。SspABCD 赋予单链和高频 PT，其中 SspB 充当切口酶，并可能引入切口以促进硫掺入。引人注目的是，SspABCD 与 SspE 结合以不寻常的方式提供针对噬菌体的保护：（1）SspE 通过其 PT 刺激的 NTPase 活性来感知序列特异性 PT，从而发挥其抗噬菌体活性，以及​​（2）SspE 通过引入抑制噬菌体增殖切口损伤损害噬菌体 DNA 复制。这些结果不仅扩展了我们对 DNA PT 修饰的多样性和功能的了解，而且增强了我们对已知防御系统库的理解。