Structural Basis for the Transcriptional Regulation of Heme Homeostasis in Lactococcus lactis
Structural Basis for the Transcriptional Regulation of Heme Homeostasis in Lactococcus lactis
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DOI：
10.1074/jbc.m112.370916
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发表时间：
2012-08-31
影响因子：
4.8
通讯作者：
Aono, Shigetoshi
中科院分区：
文献类型：
--
作者：
Sawai, Hitomi;Yamanaka, Masaru;Aono, Shigetoshi
尽管血红素是包括呼吸在内的许多生物过程的关键元素，但由于游离血红素分子的细胞毒性，应严格调节血红素稳态。许多乳酸菌，包括乳酸乳球菌，从外源获取血红素分子以建立有氧呼吸链。血红素外流系统在血红素稳态中发挥重要作用，避免获得性游离血红素的细胞毒性，但其调节机制尚不清楚。在这里，我们报道了转录调节器血红素调节转运蛋白调节器（HrtR）感知并结合血红素分子作为其生理效应器，以调节负责乳酸乳球菌血红素稳态的血红素流出系统的表达。为了阐明 HrtR 如何感知血红素分子并调节血红素流出系统基因表达的分子机制，我们分别以 2.0、3.1 和 1.9 埃的分辨率确定了 apo-HrtR.DNA 复合物、apo-HrtR 和 Holo-HrtR 的晶体结构。这些结构表明 HrtR 是转录调节因子 TetR 家族的成员。残基对 Arg-46 和 Tyr-50 对于通过氢键和 CH-pi 与 DNA 碱基相互作用的特异性 DNA 结合起着至关重要的作用。 HrtR 采用独特的机制来调节血红素感应的功能。血红素与 HrtR 的结合会导致血红素感应结构域中的 α 4 螺旋发生螺旋到螺旋的转变，从而触发 HrtR 的结构变化，导致其与靶 DNA 解离，从而解除编码血红素流出系统的基因的抑制。 HrtR 使用独特的血红素感应基序，将双组氨酸（His-72 和 His-149）连接到血红素，这对于血红素感应时 α 4 螺旋的螺旋到螺旋转变至关重要。
Although heme is a crucial element for many biological processes including respiration, heme homeostasis should be regulated strictly due to the cytotoxicity of free heme molecules. Numerous lactic acid bacteria, including Lactococcus lactis, acquire heme molecules exogenously to establish an aerobic respiratory chain. A heme efflux system plays an important role for heme homeostasis to avoid cytotoxicity of acquired free heme, but its regulatory mechanism is not clear. Here, we report that the transcriptional regulator heme-regulated transporter regulator (HrtR) senses and binds a heme molecule as its physiological effector to regulate the expression of the heme-efflux system responsible for heme homeostasis in L. lactis. To elucidate the molecular mechanisms of how HrtR senses a heme molecule and regulates gene expression for the heme efflux system, we determined the crystal structures of the apo-HrtR.DNA complex, apo-HrtR, and holo-HrtR at a resolution of 2.0, 3.1, and 1.9 angstrom, respectively. These structures revealed that HrtR is a member of the TetR family of transcriptional regulators. The residue pair Arg-46 and Tyr-50 plays a crucial role for specific DNA binding through hydrogen bonding and a CH-pi interaction with the DNA bases. HrtR adopts a unique mechanism for its functional regulation upon heme sensing. Heme binding to HrtR causes a coil-to-helix transition of the alpha 4 helix in the heme-sensing domain, which triggers a structural change of HrtR, causing it to dissociate from the target DNA for derepression of the genes encoding the heme efflux system. HrtR uses a unique heme-sensing motif with bis-His (His-72 and His-149) ligation to the heme, which is essential for the coil-to-helix transition of the alpha 4 helix upon heme sensing.