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 Hg
2+-recognition/uptake, but Cys-132 and Cys-137 were not.
Escherichia coli cells producing the MerC were hypersensitive to CdCl
2. In this case, mutation of His72 rendered the host cells less CdCl
2 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 Hg
2+ hypersensitivity and showed about 55% HgCl
2 uptake ability compared to that of the one expressing the intact
merC, indicating that the region is not essential for Hg
2+ uptake. Coexpression of
A. ferrooxidans the gene encoding mercuric reductase (
merA) and the
merC deletion mutation conferred HgCl
2 tolerance to
E. coli host cells. Under this condition, the
merC deletion gene product was exclusively present as a monomer.
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