Legatzki Antje, Grass Gregor, Anton Andreas, Rensing Christopher, Nies Dietrich H
Institut für Mikrobiologie, 06099 Halle, Germany.
J Bacteriol. 2003 Aug;185(15):4354-61. doi: 10.1128/JB.185.15.4354-4361.2003.
Cadmium and zinc are removed from cells of Ralstonia metallidurans by the CzcCBA efflux pump and by two soft-metal-transporting P-type ATPases, CadA and ZntA. The czcCBA genes are located on plasmid pMOL30, and the cadA and zntA genes are on the bacterial chromosome. Expression of zntA from R. metallidurans in Escherichia coli predominantly mediated resistance to zinc, and expression of cadA predominantly mediated resistance to cadmium. Both transporters decreased the cellular content of zinc or cadmium in this host. In the plasmid-free R. metallidurans strain AE104, single gene deletions of cadA or zntA had only a moderate effect on cadmium and zinc resistance, but zinc resistance decreased 6-fold and cadmium resistance decreased 350-fold in double deletion strains. Neither single nor double gene deletions affected zinc resistance in the presence of czcCBA. In contrast, cadmium resistance of the cadA zntA double mutant could be elevated only partially by the presence of CzcCBA. lacZ reporter gene fusions indicated that expression of cadA was induced by cadmium but not by zinc in R. metallidurans strain AE104. In the absence of the zntA gene, expression of cadA occurred at lower cadmium concentrations and zinc now served as an inducer. In contrast, expression of zntA was induced by both zinc and cadmium, and the induction pattern did not change in the presence or absence of CadA. However, expression of both genes, zntA and cadA, was diminished in the presence of CzcCBA. This indicated that CzcCBA efficiently decreased cytoplasmic cadmium and zinc concentrations. It is discussed whether these data favor a model in which the cations are removed either from the cytoplasm or the periplasm by CzcCBA.
镉和锌可通过CzcCBA外排泵以及两种软金属转运P型ATP酶（CadA和ZntA）从金属抗性罗尔斯通氏菌（Ralstonia metallidurans）细胞中去除。czcCBA基因位于质粒pMOL30上，而cadA和zntA基因位于细菌染色体上。金属抗性罗尔斯通氏菌的zntA基因在大肠杆菌中的表达主要介导对锌的抗性，而cadA的表达主要介导对镉的抗性。这两种转运蛋白均降低了该宿主细胞内锌或镉的含量。在无质粒的金属抗性罗尔斯通氏菌菌株AE104中，cadA或zntA的单基因缺失对镉和锌抗性仅有中等影响，但在双缺失菌株中锌抗性降低了6倍，镉抗性降低了350倍。在存在czcCBA的情况下，单基因或双基因缺失均不影响锌抗性。相比之下，CadA ZntA双突变体的镉抗性仅能通过CzcCBA的存在部分提高。lacZ报告基因融合表明，在金属抗性罗尔斯通氏菌菌株AE104中，cadA的表达由镉诱导而非锌诱导。在无zntA基因的情况下，cadA的表达在较低镉浓度时发生，此时锌充当诱导剂。相比之下，zntA的表达由锌和镉两者诱导，且在有无CadA的情况下诱导模式均未改变。然而，在存在CzcCBA的情况下，zntA和cadA这两个基因的表达均减弱。这表明CzcCBA有效降低了细胞质中镉和锌的浓度。文中讨论了这些数据是否支持这样一种模型，即阳离子通过CzcCBA从细胞质或周质中去除。
