DICTYOSTELIUM-DISCOIDEUM COBB MUTANTS SHOW REDUCED HEAVY-METAL ACCUMULATION ASSOCIATED WITH GENE AMPLIFICATION

Wild-type Dictyostelium discoideum cells growing on non-toxic levels o f nickel chloride or cobaltous chloride accumulate 2-3.5 times as much nickel and at least 1.5 times as much cobalt as cobB mutants. The cob B trait is dominant, confers unstable cobalt and nickel resistance and is correlated with the presence of up to 50 copies of a linear extrac hromosomal DNA, approximately 100 kb in length, derived from linkage g roup III. Independent cobB mutants can be obtained by selection on med ium containing either cobalt or nickel. The amplified DNA can be trans ferred to wild-type strains by electroporation. Strains with mutations at a second cobalt resistance locus, cobA, accumulate the same amount of cobalt, but more nickel than wild-type strains. Our results are co nsistent with the cobA mutant phenotype being due to internal sequestr ation of cobalt, and the cobB mutant phenotype being due to reduced ne t uptake of cobalt and nickel. Energy-dependent nickel export was dete ctable in wild-type and cobB mutant strains but its role in heavy meta l resistance has not yet been proved.