The merApe9 construct was first assayed for reductase activity in E. coli. The size of the zone of sensitivity around disks containing mercuric ion are shown for various strains in Table 1. The parent E. coli strain, SK1592/pPB111-47 (42), was supersensitive to mercury, producing a large ring of growth inhibition (HgSS; 30 mm). This strain has the mercury transport system and pumps even very low levels of ionic mercury into the cell, but cannot reduce Hg2+ to the nontoxic Hg°. When the supersensitive strain also contained the pNS2 plasmid, it showed full levels of Hg2+ resistance (SK1592/pPB111-47/pNS2; 14 mm). pDU202 has an intact merA gene and is the parent plasmid to pPB111-47. pNS2 alone increased slightly the resistance of wild-type E. coli strain SK1592 to Hg2+. This confirmed a recent report that R plasmids mutated in the transport functions merT and merP produce weak mercury resistance (26). These results demonstrated that merApe9 encoded a fully functional mercuric ion reductase. The sequence of the merApe9 gene in pNS2 contained a few unintended mutations, which were evidently introduced during the OE-PCR protocol. These changes produced two conservative amino acid changes in the MerApe9 protein (underlined in Fig. 1B; see Materials and Methods).