THE COBALT, ZINC, AND CADMIUM EFFLUX SYSTEM CZCABC FROM ALCALIGENES-EUTROPHUS FUNCTIONS AS A CATION-PROTON ANTIPORTER IN ESCHERICHIA-COLI

The function of the CzcABC protein complex, which mediates resistance to Co2+, Zn2+ and Cd2+ in Alcaligenes eutrophus by cation efflux, was investigated by using everted membrane vesicles of Escherichia coli an d an acridine orange fluorescence quenching assay, Since metal cation uptake could not be measured with inside-out membrane vesicles prepare d from A, eutrophus and since available E. coli strains did not expres s the Czc-mediated resistance to cobalt, zinc, and cadmium salts, muta nts off. coli which exhibited a Czc-dependent increase in heavy metal resistance were isolated, E. coli mutant strain EC351 constitutively a ccumulated Co2+, Zn2+, and Cd2+, In the presence of Czc, net uptake of these heavy metal cations was reduced to the wild-type level. Inside- out vesicles prepared from E. coli EC351 cells displayed a Czc-depende nt uptake of Co2+, Zn2+ and Cd2+ and a cation-triggered acridine orang e fluorescence increase. The czc-encoded protein complex CzcABC was sh own to be a zinc-proton antiporter.