Discovery of a haem uptake system in the soil bacterium Bradyrhizobium japonicum
Discovery of a haem uptake system in the soil bacterium Bradyrhizobium japonicum
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DOI：
10.1046/j.1365-2958.2001.02555.x
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发表时间：
2001-08-01
影响因子：
3.6
通讯作者：
Fischer, HM
中科院分区：
文献类型：
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作者：
Nienaber, A;Hennecke, H;Fischer, HM
在大豆的固氮共生体慢生根瘤菌中，我们已经确定了一个血红素吸收系统，Hmu.它由九个开放阅读框组成。这些基因的预测产物包括：外膜依赖于TonB的血红素受体HmuR；周质血红素结合蛋白HmuT；以及内膜的ABC转运蛋白HmuUV。此外，我们还鉴定了ExbBD和TonB的同源物，它们是从内膜到外膜的能量传递所必需的。突变分析和互补试验表明，HmuR和ExbBD-TonB系统，而不是HmuTUV转运蛋白，是从血红蛋白和豆类血红蛋白中摄取或获取血红素所必需的。TonB系统似乎对血红素的吸收是特异的，因为它对于铁载体的吸收是必不可少的。因此，我们提出了第二个TonB同系物的存在，该同系物在铁-螯合物的摄取中起作用。当在大豆寄主植物上测试时，hmuT-hmuR和exbD-TonB突变体表现出野生型共生特性。因此，HERN的吸收对于共生固氮并不是必不可少的，但它可能使日本芽孢杆菌在其非共生状态下获得替代铁源。转录分析和LacZ融合的表达研究表明，hmuT和hmuR的表达在低铁供应下被诱导。在毛皮和irr突变背景中也观察到了同样的情况，尽管最大诱导水平降低了。我们的结论是，要么是Fur和irr这两种调控因子独立地介导铁的转录调控，要么是一种未知的铁调控系统在缺铁条件下激活了基因表达。位于差异转录的hmuTUV和hmuR基因启动子区域的A/T丰富的顺式作用元件可能是这种类型的铁控制所必需的。
In Bradyrhizobium japonicum, the nitrogen-fixing symbiont of soybeans, we have identified a haem uptake system, Hmu, that comprises a cluster of nine open reading frames. Predicted products of these genes include: HmuR, a TonB-dependent haem receptor in the outer membrane; HmuT, a periplasmic haem-binding protein; and HmuUV, an ABC transporter in the inner membrane, Furthermore, we identified homologues of ExbBD and TonB, that are required for energy transduction from the inner to the outer membrane. Mutant analysis and complementation tests indicated that HmuR and the ExbBD-TonB system, but not the HmuTUV transporter, are essential for haem uptake or haem acquisition from haemoglobin and leghaemoglobin. The TonB system seems to be specific for haem uptake as it is dispensable for siderophore uptake. Therefore, we propose the existence of a second TonB homologue functioning in the uptake of Fe-chelates. When tested on soybean host plants, hmuT-hmuR and exbD-tonB mutants exhibited wild-type symbiotic properties. Thus, haern uptake is not essential for symbiotic nitrogen fixation but it may enable B. japonicum to have access to alternative iron sources in its non-symbiotic state. Transcript analysis and expression studies with lacZ fusions showed that expression of hmuT and hmuR is induced under low iron supply. The same was observed in fur and irr mutant backgrounds although maximal induction levels were decreased. We conclude either that both regulators, Fur and Irr, independently mediate transcriptional control by iron or that a yet unknown iron regulatory system activates gene expression under iron deprivation. An A/T-rich cis-acting element, located in the promoter region of the divergently transcribed hmuTUV and hmuR genes, is possibly required for this type of iron control.