HmuP Is a Coactivator of Irr-Dependent Expression of Heme Utilization Genes in Bradyrhizobium japonicum
HmuP Is a Coactivator of Irr-Dependent Expression of Heme Utilization Genes in Bradyrhizobium japonicum
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DOI：
10.1128/jb.00071-12
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发表时间：
2012-06-01
影响因子：
3.2
通讯作者：
O'Brian, Mark R.
中科院分区：
文献类型：
--
作者：
Escamilla-Hernandez, Rosalba;O'Brian, Mark R.
日本慢生根瘤菌利用血红素作为铁源涉及外膜血红素受体基因hmuR和血红素利用位点内的其它基因的诱导。在这里，我们发现了hmuP基因位于hmuR的上游，并与hmuTUV一起沿着转录。hmuP编码一个小的蛋白质，积累铁限制下，是转录控制的全球铁响应调节Irr，血红素利用位点内的所有基因。交联和免疫沉淀实验表明Irr在体内占据hmuR-hmuP启动子。一个hmuP突变体不生长在血红素作为铁源，但保留了使用氯化铁的能力。相应地，在铁限制下hmuR mRNA的诱导在hmuP菌株中严重减少，但Irr调节子内的其他基因不受影响。HmuP占据hmuR-hmuP启动子，因此它在基因表达中起直接调控作用。HmuP不需要Irr占用，也不是异位表达的hmuP从Irr独立的启动子足以诱导hmuR基因。因此，HmuP和Irr占据对于hmuR诱导是必需的。我们认为，HmuP是一个共激活剂的Irr依赖性表达的hmuR。
Utilization of heme as an iron source by Bradyrhizobium japonicum involves induction of the outer membrane heme receptor gene hmuR and other genes within the heme utilization locus. Here, we discovered the hmuP gene located upstream of hmuR and transcribed divergently from it along with hmuTUV. hmuP encodes a small protein that accumulated under iron limitation and is transcriptionally controlled by the global iron-responsive regulator Irr, as were all genes within the heme utilization locus. Cross-linking and immunoprecipitation experiments showed that Irr occupies the hmuR-hmuP promoter in vivo. An hmuP mutant did not grow on heme as an iron source, but retained the ability to use ferric chloride. Correspondingly, induction of hmuR mRNA under iron limitation was severely diminished in an hmuP strain, but other genes within the Irr regulon were unaffected. HmuP occupied the hmuR-hmuP promoter, and thus it plays a direct regulatory role in gene expression. HmuP was not required for Irr occupancy, nor was ectopic expression of hmuP from an Irr-independent promoter sufficient to induce the hmuR gene. Thus, both HmuP and Irr occupancy are necessary for hmuR induction. We suggest that HmuP is a coactivator of Irr-dependent expression of hmuR.