Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York, USA.
J Bacteriol. 2012 Jun;194(12):3137-43. doi: 10.1128/JB.00071-12. Epub 2012 Apr 13.
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.
根瘤菌利用血红素作为铁源涉及到外膜血红素受体基因 hmuR 和血红素利用基因座内其他基因的诱导。在这里，我们发现了 hmuP 基因，它位于 hmuR 的上游，与 hmuTUV 一起从它转录。hmuP 编码一种在铁限制下积累的小蛋白，并且像血红素利用基因座内的所有基因一样，受到全局铁反应调节因子 Irr 的转录控制。交联和免疫沉淀实验表明，Irr 在体内占据 hmuR-hmuP 启动子。hmuP 突变体不能以血红素作为铁源生长，但仍能利用氯化铁。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节作用。HmuP 不参与 Irr 占据，也不需要 Irr 独立启动子异位表达 hmuP 来诱导 hmuR 基因。因此，hmuR 的诱导既需要 HmuP 也需要 Irr 占据。我们认为 HmuP 是 Irr 依赖的 hmuR 表达的共激活因子。
