Avian pathogenic Escherichia coli (APEC), a pathotype of extraintestinal pathogenic E. coli, causes one of the most serious infectious diseases of poultry and shares some common virulence genes with neonatal meningitis-associated E. coli. TonB-dependent receptors (TBDRs) are ubiquitous outer membrane β-barrel proteins; they play an important role in the recognition of siderophores during iron uptake. Here, in the APEC strain DE205B, we investigated the role of four putative TBDRs-ireA, 0007, 0008, and 2235-in iron uptake. Glutathione-S-transferase pulldown assays indicated that the proteins encoded by these genes directly interact with TonB. Moreover, the expression levels of all four genes were significantly upregulated under iron-depleted conditions compared with iron-rich conditions. The expression levels of several iron uptake-related genes were significantly increased in the ireA, 0007, 0008, and 2235 deletion strains, with the upregulation being the most prominent in the ireA deletion mutant. Furthermore, iron uptake by the ireA deletion strain was significantly increased compared to that by the wild-type strain. Moreover, a tonB mutant strain was constructed to study the effect of tonB deletion on the TBDRs. We found that regardless of the presence of tonB, the expression levels of the genes encoding the four TBDRs were regulated by fur. In conclusion, our findings indicated that ireA, 0007, 0008, and 2235 indeed encode TBDRs, with ireA having the most important role in iron uptake. These results should help future studies explore the mechanisms underlying the TonB-dependent iron uptake pathway.

中文翻译：

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将ireA，0007、0008和2235鉴定为禽致病性大肠杆菌DE205B菌株中的TonB依赖性受体。

禽病原性大肠杆菌（APEC）是肠外病原性大肠杆菌的一种病原体，是禽类中最严重的传染病之一，并与新生儿脑膜炎相关的大肠杆菌共享一些常见的毒力基因。依赖TonB的受体（TBDR）是无处不在的外膜β-桶状蛋白。它们在铁摄取过程中对铁载体的识别中起重要作用。在这里，我们在APEC株DE205B中研究了四种假定的TBDRs-ireA，0007、0008和2235-在铁吸收中的作用。谷胱甘肽-S-转移酶的下拉分析表明这些基因编码的蛋白质直接与TonB相互作用。此外，与富铁条件相比，铁缺乏条件下所有四个基因的表达水平均显着上调。在ireA，0007、0008和2235缺失菌株中，几个与铁吸收相关的基因的表达水平显着增加，其中上调在ireA缺失突变体中最为明显。此外，与野生型菌株相比，ireA缺失菌株的铁摄取显着增加。此外，构建了一个tonB突变株，以研究tonB缺失对TBDR的影响。我们发现，无论是否存在tonB，编码4种TBDR的基因的表达水平均受fur调节。总之，我们的发现表明ireA，0007、0008和2235确实编码了TBDR，其中ireA在铁吸收中具有最重要的作用。这些结果应有助于未来的研究探索依赖于TonB的铁摄取途径的机制。