Role of N-terminal His-rich Domain of Oscillatoria brevis Bxa1 in Both Ag(I)/Cu(I) and Cd(II)/Zn(II) Tolerance

A CPx-ATPase (named Bxa1) is induced in the cyanobacterium Oscillatoria brevis upon exposure to multiple heavy metal ions. The function of the bxa1 gene was examined by heterologous expression in both Saccharomyces cerevisiae (yeast) and Escherichia coli. Expression of bxa1 in E. coli caused Ag, Cd and Zn tolerance, but in yeast became sensitive to those metals. To reveal the role of the N-terminal His-rich domain (first 35 amino acids) of Bxa1, we constructed E. coli and yeast transformants carrying the bxa1 (Δ35bxa1). The E. coli transformant with Δ35bxa1 was sensitive to heavy metals. On the other hand, the yeast Δ35bxa1 transformant increased heavy-metal tolerance than bxa1 transformant. Fluorescence microscopy suggested that the two fusion proteins Bxa1::mGFP and Δ35Bxa1::mGFP are mainly localized in yeast endoplasmic reticulum (ER). These results imply that the function of Bxa1 was lost by the N-terminus deletion in both E. coli and yeast transformants. This is the first report that the His-rich domain in O. brevis Bxa1 is essential not only to monovalent (Ag⁺ and Cu⁺) but also to divalent (Cd²⁺ and Zn²⁺) heavy metal tolerance. Moreover, we clarified the toxicity mechanism against Cd using yeast transformants.