Type II toxin–antitoxins systems are widespread in prokaryotic genomes. Typically, they comprise two proteins, a toxin, and an antitoxin, encoded by adjacent genes and forming a complex in which the enzymatic activity of the toxin is inhibited. Under stress conditions, the antitoxin is degraded liberating the active toxin. Though thousands of various toxin–antitoxins pairs have been predicted bioinformatically, only a handful has been thoroughly characterized. Here, we describe the AtaT2 toxin from a toxin–antitoxin system from Escherichia coli O157:H7. We show that AtaT2 is the first GNAT (Gcn5-related N-acetyltransferase) toxin that specifically targets charged glycyl tRNA. In vivo, the AtaT2 activity induces ribosome stalling at all four glycyl codons but does not evoke a stringent response. In vitro, AtaT2 acetylates the aminoacyl moiety of isoaccepting glycyl tRNAs, thus precluding their participation in translation. Our study broadens the known target specificity of GNAT toxins beyond the earlier described isoleucine and formyl methionine tRNAs, and suggest that various GNAT toxins may have evolved to specificaly target other if not all individual aminoacyl tRNAs.

中文翻译：

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II型GNAT毒素AtaT2抑制翻译的机制。

II 型毒素-抗毒素系统在原核基因组中广泛存在。通常，它们包含两种蛋白质，一种毒素和一种抗毒素，由相邻基因编码并形成复合物，其中毒素的酶活性受到抑制。在压力条件下，抗毒素降解释放活性毒素。尽管已经通过生物信息学预测了数千种不同的毒素-抗毒素对，但只有少数得到了彻底的表征。在这里，我们描述了来自大肠杆菌O157:H7的毒素-抗毒素系统的 AtaT2 毒素。我们表明 AtaT2 是第一个 GNAT（Gcn5 相关 N-乙酰转移酶）毒素，专门针对带电甘氨酰 tRNA。体内，AtaT2 活性在所有四个甘氨酰密码子处诱导核糖体停滞，但不会引起严格的反应。在体外，AtaT2 乙酰化异受体甘氨酰 tRNA 的氨酰基部分，从而阻止它们参与翻译。我们的研究扩大了 GNAT 毒素的已知靶标特异性，超出了早先描述的异亮氨酸和甲酰甲硫氨酸 tRNA，并表明各种 GNAT 毒素可能已经进化为专门针对其他（如果不是全部）单个氨酰 tRNA。