Type I toxin–antitoxin systems (T1TAs) are bipartite bacterial loci encoding a growth-inhibitory toxin and an antitoxin small RNA (sRNA). In many of these systems, the transcribed toxin mRNA is translationally inactive, but becomes translation-competent upon ribonucleolytic processing. The antitoxin sRNA targets the processed mRNA to inhibit its translation. This two-level control mechanism prevents cotranscriptional translation of the toxin and allows its synthesis only when the antitoxin is absent. Contrary to this, we found that the timP mRNA of the timPR T1TA locus does not undergo enzymatic processing. Instead, the full-length timP transcript is both translationally active and can be targeted by the antitoxin TimR. Thus, tight control in this system relies on a noncanonical mechanism. Based on the results from in vitro binding assays, RNA structure probing, and cell-free translation experiments, we suggest that timP mRNA adopts mutually exclusive structural conformations. The active form uniquely possesses an RNA pseudoknot structure which is essential for translation initiation. TimR preferentially binds to the active conformation, which leads to pseudoknot destabilization and inhibited translation. Based on this, we propose a model in which “structural processing” of timP mRNA enables tight inhibition by TimR in nonpermissive conditions, and TimP synthesis only upon TimR depletion.

中文翻译：

---

RNA 假结介导毒素翻译和抗毒素抑制


I 型毒素-抗毒素系统 (T1TA) 是编码生长抑制毒素和抗毒素小 RNA (sRNA) 的二分细菌位点。在许多此类系统中，转录的毒素 mRNA 在翻译上无翻译活性，但在核糖核酸水解加工后变得具有翻译能力。抗毒素 sRNA 以加工后的 mRNA 为目标，抑制其翻译。这种两级控制机制可防止毒素的共转录翻译，并仅在抗毒素不存在时才允许其合成。与此相反，我们发现 timPR T1TA 位点的 timP mRNA 不经过酶处理。相反，全长 timP 转录物具有翻译活性，并且可以被抗毒素 TimR 靶向。因此，该系统的严格控制依赖于非规范机制。根据体外结合测定、RNA结构探测和无细胞翻译实验的结果，我们建议timP mRNA采用互斥的结构构象。活性形式独特地具有RNA假结结构，这对于翻译起始至关重要。 TimR 优先与活性构象结合，导致假结不稳定并抑制翻译。基于此，我们提出了一个模型，其中 timP mRNA 的“结构加工”使得 TimR 在非允许条件下能够紧密抑制，并且只有在 TimR 耗尽时才能合成 TimP。