Structure and function of plasmids governing resistance to heavy metals in Alcaligenes eutrophus CH34

The facultative chemolithotroph, Alcaligenes eutrophus CH34 was found in polluted sediments of a zinc factory and displays plasmid bound resistance to cadmium, cobalt, mercury, nickel, zinc and copper ions. Resistance to heavy metals is carried by two large transferable plasmids: pMOL28 and pMOL30. Resistance to Hg is controlled in both plasmids by a Hg-transposon (Tn4378 and Tn4380 respectively). Resistance to Cd, Co and Zn is coded by a 9 kb fragment (ccz⁺). Transposons and plasmids were compared by hybridisation techniques and physical maps were constructed. Various mutants and derivatives of these plasmids have been studied. Hg transposons and the ccz⁺ fragment were used as probes in ecological studies and in the search for new resistant strains. The resistance mechanisms of CH34 against Cd, Co and Zn are being studied. One aspect of the resistance mechanism is the accumulation of Cd and Zn by CH34. There are at least three phenomena: (1) a plasmid controlled Cd⁺⁺ resistance system, which was inducible by 0.1 mM of Cd; (2) a plasmid independent Cd⁺⁺ binding system where a phosphatase seems to be involved in the production of Cd HPO₄ which will probably bind to the lipopolysaccharide layer; (3) the formation of Cd CO₃ which is highly dependant from the carbon source and its amount. The accumulation of these metals can lead to some possible applications in cleaning effluents contaminated by heavy metals.