The hmuR operon encodes proteins for the uptake and utilization of heme as a nutritional iron source in Bradyrhizobium japonicum. The hmuR operon is transcriptionally activated by the Irr protein and is also positively controlled by HmuP by an unknown mechanism. An hmuP mutant does not express the hmuR operon genes nor does it grow on heme. Here, we show that hmuR expression from a heterologous promoter still requires hmuP, suggesting that HmuP does not regulate at the transcriptional level. Replacement of the 5′ untranslated region (5′UTR) of an HmuP‐independent gene with the hmuR 5′UTR conferred HmuP‐dependent expression on that gene. Recombinant HmuP bound an HmuP‐responsive RNA element (HPRE) within the hmuR 5′UTR. A 2 nt substitution predicted to destabilize the secondary structure of the HPRE abolished both HmuP binding activity in vitro and hmuR expression in cells. However, deletion of the HPRE partially restored hmuR expression in an hmuP mutant, and it rescued growth of the hmuP mutant on heme. These findings suggest that the HPRE is a negative regulatory RNA element that is suppressed when bound by HmuP to express the hmuR operon.

中文翻译：

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Bradyrhizobium japonicumHmuP 是一种 RNA 结合蛋白，通过抑制 5' 非翻译区的负调控 RNA 元件来正向控制 hmuR 操纵子的表达


hmuR 操纵子编码用于吸收和利用血红素作为日本慢生根瘤菌中的营养铁源的蛋白质。 hmuR 操纵子由 Irr 蛋白转录激活，并且还通过未知机制受到 HmuP 的正向控制。 hmuP 突变体不表达 hmuR 操纵子基因，也不在血红素上生长。在这里，我们表明来自异源启动子的 hmuR 表达仍然需要 hmuP，这表明 HmuP 不在转录水平上进行调节。用 hmuR 5'UTR 替换 HmuP 独立基因的 5' 非翻译区 (5'UTR) 赋予该基因 HmuP 依赖性表达。重组 HmuP 在 hmuR 5'UTR 内结合 HmuP 响应 RNA 元件 (HPRE)。预计会破坏 HPRE 二级结构稳定性的 2 nt 取代消除了体外 HmuP 结合活性和细胞中 hmuR 表达。然而，HPRE 的缺失部分恢复了 hmuP 突变体中 hmuR 的表达，并挽救了 hmuP 突变体在血红素上的生长。这些发现表明 HPRE 是一种负调节 RNA 元件，当与 HmuP 结合以表达 hmuR 操纵子时，该元件会受到抑制。