GENES FOR ALL METALS - A BACTERIAL VIEW OF THE PERIODIC-TABLE - THE-1996-THOM-AWARD-LECTURE

Bacterial chromosomes have genes for transport proteins for inorganic nutrient cations and oxyanions, such as NH4+, K+, Mg2+, Co2+, Fe3+, Mn 2+, Zn2+ and other trace cations, and PO43-, SO42- and less abundant o xyanions, Together these account for perhaps a few hundred genes in ma ny bacteria, Bacterial plasmids encode resistance systems for toxic me tal and metalloid ions including Ag+, AsO2-, AsO43-, Cd2+, Co2+, CrO42 -, Cu2+, Hg2+, Ni2+, Pb2+, TeO32-, TI+ and Zn2+. Most resistance syste ms function by energy-dependent efflux of toxic ions. A few involve en zymatic (mostly redox) transformations. Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. The Cd2+-resistance cation pump of Gram-positive bacteria is membrane P-type ATPase, which has been labeled with P-32 from [gamma-P-32]ATP and drives ATP-dependent Cd2+ (and Zn2+) transport by membrane vesicle s. The genes defective in the human hereditary diseases of copper meta bolism, Menkes syndrome and Wilson's disease, encode P-type ATPases th at are similar to bacterial cadmium ATPases, The arsenic resistance sy stem transports arsenite [As(III)], alternatively with the ArsB polype ptide functioning as a chemiosmotic efflux transporter or with two pol ypeptides, ArsB and ArsA, functioning as an ATPase, The third protein of the arsenic resistance system is an enzyme that reduces intracellul ar arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system, In Gram-negative cells, a three polypeptide complex functions as a chemiosmotic cation/protein exchanger to efflux Cd2+, Zn2+ and C o2+. This pump consists of an inner membrane (CzcA), an outer membrane (CzcC) and a membrane-spanning (CzcB) protein that function together.