Comparative study on Ni2+-affinity transport of nickel/cobalt permeases (NiCoTs) and the potential of recombinant Escherichia coli for Ni2+ bioaccumulation

Comparative evaluation on Ni2+-uptake of two nickel-affinity transmembrane proteins (NiCoTs) respectively from Helocobacter pylori (NixA) and Staphylococcus aureus (NisA) was performed. Expression of NiCoTs alone did not promote Ni2+ uptake of the recombinant strains and made the growth susceptible to Ni2+. However, recombinant strains expressing both NiCoTs and Metallothionein (MT) showed enhanced tolerance to Ni2+ and Ni2+ uptake. The maximum Ni2+-uptake capacity of recombinant strain N1c expressing NixA+MT reached 83.33 mg g−1, higher than 45.45 mg g−1 of recombinant strain N1d expressing NisA+MT. N1c exhibited more effective Ni2+ accumulation than N1d in the presence of Na+, Co2+ and Cd2+. NiCoTs promoted intracellular Ni2+ uptake of the recombinant strains. Phosphate groups dominated Ni2+ binding of wild type Escherichia coli, but carboxyl groups contributed more for N1c and N1d. The result suggested that NixA has a higher specificity in Ni2+ binding than NisA, and both NiCoTs and MT are important for Ni2+ bioaccumulation.