CLONING AND EXPRESSION IN ESCHERICHIA-COLI OF MERCURIC ION RESISTANCE CODING GENES FROM ZYMOMONAS-MOBILIS

From a genomic library of Zymomonas mobilis prepared in Escherichia coli, two clones (carrying pZH4 and pZH5) resistant to the mercuric ion were isolated. On partial restriction analysis these two clones appeared to have the same 2.9 kb insert. Mercuric reductase activity was assayed from the Escherichia coli clone carrying pZH5 and it was Hg2+-inducible, NADH dependent and also required 2-mercaptoethanol for its activity. The plasmid pZH5 encoded three polypeptides, mercuric reductase (merA; 65 kDa), a transport protein (merT 18-17 kDa) and merC (15 kDa) as analysed by SDS-PAGE. Southern blot analysis showed the positive signal for the total DNA prepared from Hg(r) Z. mobilis but not with the Hg(s) strain which was cured for a plasmid (30 kb). These results were also confirmed by isolating this plasmid from Hg(r) Z. mobilis and transforming into E. coli. Moreover the plasmid pZH5 also hybridized with the mer probes derived from Tn21.