Cell-free mercury(II)-reducing activity in a plasmid-bearing strain of Escherichia coli

The ability to reduce Hg(II) to Hg(0), which is determined by a plasmid-borne gene in Escherichia coli, is conferred by a Hg(II)-inducible activity which is located in the cytoplasm rather than in the periplasmic space of the cell. This Hg(II)-reducing activity can be isolated from the supernatant of a 160,000 x g centrifugation after French Press disruption of the cells. The activity is dependent on glucose-6-phosphate, glucose-6-phosphate dehydrogenase, and 2-mercaptoethanol, but is not enhanced by added nicotinamide adenine dinucleotide phosphate. Treatment of the active fraction with N-ethylmaleimide causes irreversible loss of the Hg(II)-reducing activity. Unlike the Hg(II)-reducing activity found in intact cells, the cell-free activity is not inhibited by toluene, potassium cyanide, or m-chlorocarbonylcyanide-phenylhydrazone; however, it is inhibited by Ag(I) and phenylmercuric acetate to the same extent as the activity in intact cells. Neither phenylmercuric acetate nor methylmercuric chloride is reduced to Hg(0) by the cell-free activity. Au(III), however, is a substrate for the cell-free activity; it is reduced to metallic colloidal Au(0).