Mercury resistance determinants in bacteria.are often plasmid-borne or transposon-mediated. Mercuric reductase, one of the proteins encoded by the mercury resistance operon, catalyses a unique reaction in which mercuric ions, Hg (II), are reduced t.o mercury, metal Hg(O) using NADPH as a source of reducing power. Mercuric reductase was purified from Azotobacter chroococcum SS_2 using Red A dye matrix affinity chromatography. Freshly purified preparations of the enzyme showed a single band on polyacrylamide gel electrophoresis under non-denaturing conditions. After SDS-polyacrylamide gel electrophoresis of the freshly prepared enzyme, two protein bands, a major and a minor one, were observed with molecular weight 69 000 and 54 000, respectively. The molecular weight of the native enzyme as determined by gel filtration in Sephacryl S-300 was 142 000. The Km of Hg~(2+)-reductase for HgCl_2 was 11.11 #mu#mol l~(-1). Titration with 5,5'-dithiobis (2-nitrobenzoate) demonstrated that two enzyme-SH groups become kinetically accessible on reduction with NADPH.
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