The ligation mixture was transformed into a "wild-type" E. coli strain, SK1592, containing the pPB111-47 plasmid, which encodes the mer operon, has mercury transport functions, but has a disruption in the merA gene. Ampicillin-resistant transformants were selected and replica plated onto media containing 100 and 200 tLM Hg2+. One colony that grew exceptionally well on 200 ,uM Hg2+ was characterized further and contained merApe9 in plasmid pNS2 (Table 1 and Materials
and Methods).