THE ROLE OF CYSTEINE RESIDUES IN THE TRANSPORT OF MERCURIC IONS BY THE TN501 MERT AND MERP MERCURY-RESISTANCE PROTEINS

Each cysteine residue in the MerT and MerP polypeptides of bacterial transposon Tn501 was replaced by serine, and the mercury-resistance phenotypes of the mutants were determined in Escherichia coli. Cys-24 and Cys-25 in the first transmembrane region of MerT were essential for transport of mercuric ions through the cytoplasmic membrane, and mutations Cys-76-Ser, Cys-82-Ser or Gly-38-Asp in MerT or Cys-36-Ser in MerP all reduced transport and resistance. Deletion of the merP gene slightly reduced mercuric ion resistance and transport, whereas a Cys-33-Ser mutation in MerP appears to block transport of mercuric ions by MerT. The effects of deleting merP on mutations in merT were tested. The 116-amino-acid MerT protein is sufficient for mercuric ion transport across the cytoplasmic membrane.