Resistance to Hg(2+) in bacteria appears to be governed by a specialized system which includes mercuric reductase. MerA protein is responsible for volatilizing mercury as Hg(0). H(+) + Hg + NADP(+) = Hg(2+) + NADPH Binds 1 FAD per subunit. Homodimer. The active site is a redox-active disulfide bond. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.