Functional Dissection of a Mercuric Ion Transporter, MerC, from Acidithiobacillus ferrooxidans

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Topological analysis with a phoA gene fusion suggested that Acidithiobacillus ferrooxidans MerC, a mercury transporter, has two periplasmic loops and four transmembrane domains. Cys-23 and Cys-26 of the protein were involved in Hg²⁺-recognition/uptake, but Cys-132 and Cys-137 were not. Escherichia coli cells producing the MerC were hypersensitive to CdCl₂. In this case, mutation of His72 rendered the host cells less CdCl₂ sensitive, whereas none of the Cys residues affected it. E. coli cells expressing the gene encoding a mercuric ion transporter (merC)-deletion mutant, in which the coding-sequence of the carboxy-terminal cytoplasmic region was removed, retained Hg²⁺ hypersensitivity and showed about 55% HgCl₂ uptake ability compared to that of the one expressing the intact merC, indicating that the region is not essential for Hg²⁺ uptake. Coexpression of A. ferrooxidans the gene encoding mercuric reductase (merA) and the merC deletion mutation conferred HgCl₂ tolerance to E. coli host cells. Under this condition, the merC deletion gene product was exclusively present as a monomer.