BACTERIAL RESISTANCES TO TOXIC METAL-IONS - A REVIEW

Bacterial plasmids encode resistance systems for toxic metal ions, inc luding Ag+, AsO2-, AsO43-, Cd2+, Co2+, CrO43-, Cu2+ Hg2+, Ni2+, Pb2+, Sb3+, TeO32-, Tl+ and Zn2+. The function of most resistance systems is based on the energy-dependent efflux of toxic ions. Some of the efflu x systems are ATPases and others are chemiosmotic cation/proton antipo rters. The Cd2+-resistance ATPase of Gram-positive bacteria (CadA) is membrane cation pump homologous with other bacterial, animal and plant P-type ATPases. CadA has been labeled with P-32 from [alpha-P-32]ATP and drives ATP-dependent Cd2+ (and Zn2+) uptake by inside-out membrane vesicles (equivalent to efflux from whole cells). Recently, isolated genes defective in the human hereditary diseases of copper metabolism, namely Menkes syndrome and Wilson's disease, encode P-type ATPases th at are more similar to bacterial CadA than to other ATPases from eukar yotes. The arsenic resistance efflux system transports arsenite [As(II I)], alternatively using either a double-polypeptide (ArsA and ArsB) A TPase or a single-polypeptide (ArsB) functioning as a chemiosmotic tra nsporter. The third gene in the arsenic resistance system, arsC, encod es an enzyme that converts intracellular arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system. The triple-polypeptide Czc (Cd2+, Zn2+ and Co2+) chemiosmotic efflux pump consists of inner m embrane (CzcA), outer membrane (CzcC) and membrane-spanning (CzcB) pro teins that together transport cations from the cytoplasm across the pe riplasmic space to the outside of the cell.