Role of MerT and MerP from Pseudomonas K-62 plasmid pMR26 in the transport of phenylmercury.

To investigate the individual role of MerT and MerP encoded by Pseudomonas K-62 pMR26 in the transport of phenylmercury, a series of mutants with a specific point mutation in merT and/or genetic deletion in merP were constructed and transformed into Escherichia coli XL1-Blue. Transport of phenylmercury across the cytoplasmic membrane of E. coli mediated by MerT and MerP was inhibited by NaCN and by cold temperatures. Deletion of merP reduced, but did not completely abolish the C6H5Hg+-hyperuptake and -hypersensitive phenotypes suggesting that transport of phenylmercury into the cytoplasm of E. coli is still occurring. Mutations of the vicinal cysteine residues (Cys24 and Cys25) in the first transmembrane region of MerT to serine caused complete loss of Hg2+-hyperuptake and -hypersensitivity, whereas the mutations did not affect the C6H5Hg+-hyperuptake and -hypersensitive phenotypes. In addition, no additive effect on the C6H5Hg+-hyperuptake and -hypersensitive phenotypes was found, when mutations of the two cysteines in MerT to serine were further introduced in the merP-deleted mutants. These results clearly demonstrated that the vicinal cysteine residues of MerT are not involved in the transport of C6H5Hg+, but indeed are involved in the transport of Hg2+ as previously reported.