Copper, Nickel, And Zinc Cations Biosorption Properties Of Gram-positive And Gram-negative MerP Mercury-resistance Proteins

ABSTRACT The mercury resistance (mer) operon is present in a wide range of bacteria but relevant research has been conducted mainly on Gram-negative bacteria. We compared the heavy metal biosorption properties of Gram-positive and Gram-negative MerP, a key metal-binding transporter of the mer operon that scavenges heavy metal ions in the periplasm for subsequent neutralization in the cytoplasm by other Mer proteins. The MerP proteins of the Gram-positive Bacillus cereus RC607 and Gram-negative Pseudomonas aeruginosa K-62 strains in Escherichia coli strain BL21 were expressed to derive the recombinant strains BL21+MerPBc and BL21+MerPPa, respectively. The BL21+MerPBc having a higher affinity for Ni2+ and Cu2+ ions than that of Zn2+ ions was found, whereas BL21+MerPPa preferentially adsorbed Zn2+ ions over Ni2+ and Cu2+ ions. Furthermore, X-ray photoelectron spectroscopy revealed that regardless of the initial ion concentration, Zn, Ni, and Cu were mainly present as oxides in BL21+MerPBc but were mainly present as sulfides at low ion concentrations in BL21+MerPPa, with oxides primarily detected only when the initial ion concentrations were high. These findings were confirmed through X-ray absorption near edge structure and extended X-ray absorption fine structure analyses. These results have revealed that the heavy metal biosorption properties differ between Gram-positive and Gram-negative MerP proteins.