Bacterial abortive infection (Abi) systems are type IV toxin–antitoxin (TA) system, which could elicit programmed cell death and constitute a native survival strategy of pathogenic bacteria under various stress conditions. However, no rhizobial AbiE family TA system has been reported so far. Here, a M. huakuii AbiE TA system was identified and characterized. A mutation in M. huakuii abiEi gene, encoding an adjacent GntR-type transcriptional regulator, was generated by homologous recombination. The abiEi mutant strain grew less well in rich TY medium, and displayed increased antioxidative capacity and enhanced gentamicin resistance, indicating the abiEi operon was negatively regulated by the antitoxin AbiEi in response to the oxidative stress and a particular antibiotic. The mRNA expression of abiEi gene was significantly up-regulated during Astragalus sinicus nodule development. The abiEi mutant was severely impaired in its competitive ability in rhizosphere colonization, and was defective in nodulation with 97% reduction in nitrogen-fixing capacity. The mutant infected nodule cells contained vacuolation and a small number of abnormal bacteroids with senescence character. RNA-seq experiment revealed it had 5 up-regulated and 111 down-regulated genes relative to wild type. Of these down-regulated genes, 21 are related to symbiosis nitrogen fixation and nitrogen mechanism, 16 are involved in the electron transport chain and antioxidant responses, and 12 belong to type VI secretion system (T6SS). M. huakuii AbiEi behaves as a key transcriptional regulator mediating root nodule symbiosis.

中文翻译：

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Mesorhizobium huakuii 转录调节因子 AbiEi 在结瘤中起关键作用，对细菌应激反应很重要

细菌性流产感染（Abi）系统是IV型毒素-抗毒素（TA）系统，可引发程序性细胞死亡，构成病原菌在各种应激条件下的天然生存策略。然而，到目前为止，还没有报道过根瘤菌 AbiE 家族 TA 系统。在这里，鉴定和表征了 M. huakuii AbiE TA 系统。M. huakuii abiEi 基因中的一个突变，编码相邻的 GntR 型转录调节因子，是通过同源重组产生的。abiEi 突变株在富含TY 培养基中生长较差，抗氧化能力增强，庆大霉素抗性增强，表明abiEi 操纵子受到抗毒素AbiEi 对氧化应激和特定抗生素的负调控。abiEi基因的mRNA表达在黄芪结节发育过程中显着上调。abiEi 突变体在根际定殖中的竞争能力严重受损，结瘤有缺陷，固氮能力降低了 97%。突变感染的结节细胞含有空泡和少量具有衰老特征的异常类杆菌。RNA-seq 实验显示，与野生型相比，它有 5 个上调基因和 111 个下调基因。在这些下调的基因中，21个与共生固氮和氮机制有关，16个参与电子传递链和抗氧化反应，12个属于VI型分泌系统（T6SS）。M. huakuii AbiEi 是介导根瘤共生的关键转录调节因子。