Through subcultivations of Thiobacillus thiooxidans WU-79A in autotrophic media in which the concentrations of Cd2+ and Zn2+ were increased successively, Cd2+-resistant (CDR) and Zn2+-resistant strains (ZNR) were obtained. The growth of WU-79A was inhibited by the addition of 25 mM Cd2+ as well as Zn2+. However, CDR and ZNR could grow without any lag phase in media containing 200 mM Cd2+ and 250 mM Zn2+, respectively. CDR and ZNR were able to grow even in media containing up to 400 mM Cd2+ and 600 mM Zn2+, respectively, although they exhibited lag phases. CDR could grow in medium containing up to 250 mM Zn2+, as could ZNR in medium containing up to 200 mM Cd2+. Cd2+-binding and Zn2+-binding proteins were isolated from CDR and ZNR, respectively, by gel filtration and ion exchange chromatography. The molecular weights of both proteins were estimated to be approximately 13,000 by gel filtration. The fact that there was no strong absorption at 280 nm of the proteins suggested that they had few aromatic amino acids. Broad absorption bands which are typical of mercaptide (metal thiolate) complexes were detected. The properties of the proteins were spectrophotometrically similar to those of metallothionein.
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