Nickel ions are crucial components for the catalysis of biological reactions in prokaryotic organisms. As an uncontrolled nickel trafficking is toxic for living organisms, nickel-dependent bacteria have developed tightly regulated strategies to maintain the correct intracellular metal ion quota. These mechanisms require transcriptional regulator proteins that respond to nickel concentration, activating or repressing the expression of specific proteins related to Ni(II) metabolism. In Streptomyces griseus, a Gram-positive bacterium used for antibiotic production, SgSrnR and SgSrnQ regulate the nickel-dependent antagonistic expression of two superoxide dismutase (SOD) enzymes, a Ni-SOD and a FeZn-SOD. According to a previously proposed model, SgSrnR and SgSrnQ form a protein complex in which SgSrnR works as repressor, binding directly to the promoter of the gene coding for FeZn-SOD, while SgSrnQ is the Ni(II)-dependent co-repressor. The present work focuses on the determination of the biophysical and functional properties of SgSrnR. The protein was heterologously expressed and purified from Escherichia coli. The structural and metal-binding analysis, carried out by circular dichroism, light scattering, fluorescence and isothermal titration calorimetry, showed that the protein is a well-structured homodimer, able to bind nickel with moderate affinity. DNase I footprinting and β-galactosidase gene reporter assays revealed that apo-SgSrnR is able to bind its DNA operator and activates a transcriptional response. The structural and functional properties of this protein are discussed relatively to its role as a Ni(II)-dependent sensor.

中文翻译：

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来自灰色链霉菌的 SrnR 是一种镍结合转录激活剂。

镍离子是催化原核生物生物反应的关键成分。由于不受控制的镍贩运对生物体有毒，依赖镍的细菌已经制定了严格监管的策略，以维持正确的细胞内金属离子配额。这些机制需要转录调节蛋白来响应镍浓度，激活或抑制与 Ni(II) 代谢相关的特定蛋白质的表达。在用于抗生素生产的革兰氏阳性菌Streptomyces griseus 中，Sg SrnR 和Sg SrnQ 调节两种超氧化物歧化酶 (SOD) 酶（Ni-SOD 和 FeZn-SOD）的镍依赖性拮抗表达。根据先前提出的模型，SgSrnR 和Sg SrnQ 形成一个蛋白质复合物，其中Sg SrnR 作为阻遏物，直接与编码 FeZn-SOD 的基因的启动子结合，而Sg SrnQ 是 Ni(II) 依赖性共阻遏物。目前的工作重点是确定Sg SrnR的生物物理和功能特性。该蛋白质是从大肠杆菌中异源表达和纯化的。通过圆二色性、光散射、荧光和等温滴定量热法进行的结构和金属结合分析表明，该蛋白质是一种结构良好的同型二聚体，能够以中等亲和力结合镍。DNase I 足迹和 β-半乳糖苷酶基因报告分析表明，apo- SgSrnR 能够结合其 DNA 操纵子并激活转录反应。相对于其作为 Ni(II) 依赖性传感器的作用，讨论了该蛋白质的结构和功能特性。