Unlike nucleobase modifications in canonical restriction-modification systems, DNA phosphorothioate (PT) epigenetic modification occurs in the DNA sugar-phosphate backbone when the nonbridging oxygen is replaced by sulfur in a double-stranded (ds) or single-stranded (ss) manner governed by DndABCDE or SspABCD, respectively. SspABCD coupled with SspE constitutes a defense barrier in which SspE depends on sequence-specific PT modifications to exert its antiphage activity. Here, we identified a new type of ssDNA PT-based SspABCD-SspFGH defense system capable of providing protection against phages through a mode of action different from that of SspABCD-SspE. We provide further evidence that SspFGH damages non-PT-modified DNA and exerts antiphage activity by suppressing phage DNA replication. Despite their different defense mechanisms, SspFGH and SspE are compatible and pair simultaneously with one SspABCD module, greatly enhancing the protection against phages. Together with the observation that the sspBCD-sspFGH cassette is widely distributed in bacterial genomes, this study highlights the diversity of PT-based defense barriers and expands our knowledge of the arsenal of phage defense mechanisms.

中文翻译：

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SspABCD-SspFGH 构成了一种新型的基于 DNA 硫代磷酸酯的细菌防御系统

与经典限制性修饰系统中的核碱基修饰不同，当非桥接氧以双链 (ds) 或单链 (ss) 方式被硫取代时，DNA 硫代磷酸酯 (PT) 表观遗传修饰发生在 DNA 糖-磷酸骨架中分别由 DndABCDE 或 SspABCD。SspABCD 与 SspE 结合构成​​了一道防御屏障，其中 SspE 依赖于序列特异性 PT 修饰来发挥其抗噬菌体活性。在这里，我们确定了一种新型的基于 ssDNA PT 的 SspABCD-SspFGH 防御系统，能够通过与 SspABCD-SspE 不同的作用模式提供针对噬菌体的保护。我们提供了进一步的证据表明 SspFGH 会破坏非 PT 修饰的 DNA 并通过抑制噬菌体 DNA 复制来发挥抗噬菌体活性。尽管他们的防御机制不同，SspFGH 和 SspE 兼容并与一个 SspABCD 模块同时配对，大大增强了对噬菌体的保护。连同观察到的sspBCD-sspFGH盒广泛分布在细菌基因组中，这项研究突出了基于 PT 的防御屏障的多样性，并扩展了我们对噬菌体防御机制库的了解。