There are two distinct nickel resistance loci on plasmid pTOM9 from Achromo
bacter xylosoxidans 31A, ncc and nre. Expression of the nreB gene was speci
fically induced by nickel and conferred nickel resistance on both A. xyloso
xidans 31A and Escherichia coli. E. coli cells expressing nreB showed reduc
ed accumulation of Ni2+, suggesting that NreB mediated nickel efflux. The h
istidine-rich C-terminal region of NreB mas not essential but contributed t
o maximal Ni2+ resistance.