Contact-dependent growth inhibition (CDI) is a form of interbacterial competition mediated by CdiB-CdiA two-partner secretion systems. CdiA effector proteins carry polymorphic C-terminal toxin domains (CdiA-CT), which are neutralized by specific CdiI immunity proteins to prevent self-inhibition. Here, we present the crystal structures of CdiA-CT⋅CdiI complexes from Klebsiella pneumoniae 342 and Escherichia coli 3006. The toxins adopt related folds that resemble the ribonuclease domain of colicin D, and both are isoacceptor-specific tRNases that cleave the acceptor stem of deacylated tRNAGAUIle. Although the toxins are similar in structure and substrate specificity, CdiA-CTKp342 activity requires translation factors EF-Tu and EF-Ts, whereas CdiA-CTEC3006 is intrinsically active. Furthermore, the corresponding immunity proteins are unrelated in sequence and structure. CdiIKp342 forms a dimeric β sandwich, whereas CdiIEC3006 is an α-solenoid monomer. Given that toxin-immunity genes co-evolve as linked pairs, these observations suggest that the similarities in toxin structure and activity reflect functional convergence.

中文翻译：

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细菌tRNase毒素中Barnase / EndoU / Colicin / RelE（BECR）折叠的趋同进化。

接触依赖性生长抑制（CDI）是由CdiB-CdiA两伙伴分泌系统介导的细菌竞争的一种形式。CdiA效应蛋白带有多态C端毒素域（CdiA-CT），该域被特定的CdiI免疫蛋白中和以防止自我抑制。在这里，我们介绍了肺炎克雷伯氏菌342和大肠杆菌3006的CdiA-CT⋅CdiI复合物的晶体结构。毒素采用类似于大肠菌素D的核糖核酸酶结构域的相关折叠，并且都是同种受体特异性tRNase，它们裂解了受体的茎。去酰化的tRNAGAUIle。尽管毒素的结构和底物特异性相似，但CdiA-CTKp342的活性需要翻译因子EF-Tu和EF-Ts，而CdiA-CTEC3006具有内在的活性。此外，相应的免疫蛋白在序列和结构上是无关的。CdiIKp342形成二聚体β三明治，而CdiIEC3006是α-电磁体单体。鉴于毒素免疫基因以链接对的形式共同进化，这些观察结果表明毒素结构和活性的相似性反映了功能趋同。