Polyphosphate produced in recombinant Escherichia coli confers mercury resistance

An Escherichia coli strain was generated by fusion of a merA-deleted broad-spectrum mer operon from Pseudomonas K-62 with a bacterial polyphosphate kinase gene (ppk) from Klebsiella aerogenes in vector pUC119. A large amount of the ppk-specified polyphosphate was identified in the mercury-induced bacterium with the fusion plasmid designated pMKB18 but not in the cells without mercury induction. These results suggest that the synthesis of polyphosphate as well as the expression of the mer genes is mercury-inducible and regulated by merR. The E. coli strain with pMKB18 was more resistant to both Hg2+ and C6H5Hg+ than its isogenic strain with cloning vector pUC119. The recombinant strain accumulated more mercury from Hg2+- and C6H5Hg+-contaminated medium. Hg2+ transported into the cytoplasm appeared to be bound by chelation with the polyphosphate produced by the recombinant cells. The transported phenylmercury was degraded to Hg2+ before the chelation since polyphosphate did not directly chelate with C6H5Hg+. These results indicate that polyphosphate is capable of reducing the cytotoxicity of the transported Hg2+ probably via chelation between polyphosphate and Hg2+.