Eitinger T, Mandrand-Berthelot M A
Humboldt-Universität zu Berlin, Institut für Biologie/Mikrobiologie, Germany.
Arch Microbiol. 2000 Jan;173(1):1-9. doi: 10.1007/s002030050001.
The transition metal Ni is an essential cofactor for a number of enzymatic reactions in both prokaryotes and eukaryotes. Molecular analyses have revealed the existence of two major types of high-affinity Ni2+ transporters in bacteria. The Nik system of Escherichia coli is a member of the ABC transporter family and provides Ni2+ ion for the anaerobic biosynthesis of hydrogenases. The periplasmic binding protein of the transporter, NikA, is likely to play a dual role. It acts as the primary binder in the uptake process and is also involved in negative chemotaxis to escape Ni overload. Expression of the nik operon is controlled by the Ni-responsive repressor NikR, which shows functional similarity to the ferric ion uptake regulator Fur. The second type of Ni2+ transporter is represented by HoxN of Ralstonia eutropha, the prototype of a novel family of transition metal permeases. Members of this family have been identified in gram-negative and gram-positive bacteria and recently also in a fission yeast. They transport Ni2+ with very high affinity, but differ with regard to specificity. Site-directed mutagenesis experiments have identified residues that are essential for transport. Besides these uptake systems, different types of metal export systems, which prevent microorganisms from the toxic effects of Ni2+ at elevated intracellular concentrations, have also been described.
过渡金属镍是原核生物和真核生物中许多酶促反应所必需的辅助因子。分子分析表明，细菌中存在两种主要类型的高亲和力镍离子转运蛋白。大肠杆菌的Nik系统是ABC转运蛋白家族的成员，为氢化酶的厌氧生物合成提供镍离子。该转运蛋白的周质结合蛋白NikA可能发挥双重作用。它在摄取过程中作为主要结合蛋白，还参与负趋化作用以避免镍过载。nik操纵子的表达受镍响应阻遏物NikR控制，NikR与铁离子摄取调节因子Fur具有功能相似性。第二类镍离子转运蛋白以嗜麦芽窄食单胞菌的HoxN为代表，它是一类新型过渡金属通透酶家族的原型。该家族成员已在革兰氏阴性菌和革兰氏阳性菌中被鉴定出来，最近在一种裂殖酵母中也有发现。它们以非常高的亲和力转运镍离子，但在特异性方面有所不同。定点诱变实验已确定了转运所必需的残基。除了这些摄取系统外，还描述了不同类型的金属输出系统，这些系统可防止微生物在细胞内镍离子浓度升高时受到其毒性影响。
