Metallothioneins, a class of low molecular weight cysteine-rich proteins that bind heavy metal ions, have been found in various eucaryotic organisms. When fission yeasts are grown in the presence of high concentration of CdCl2, large amounts of Cd-binding peptides (Cd-BP1 and Cd-BP2) are synthesized. Cd-BP1 (MW 4000) contains 4 mole of small unit peptide (cadystin, MW 771), 6 mole of Cd2+, and 1 mole of the labile sulfide; on the other hand, Cd-BP2 (MW 1800) contains 2 mole of cadystin and 2 mole of Cd2+. While Cd-BP2 shows similarities to mammalian Cd-thioneins in UV and CD spectra, Cd-BP1 has a characteristic shoulder at 265 nm in the UV absorption spectrum and shows two marked Cotton bands at 257 nm (negative) and 275 nm (positive). These characteristics of Cd-BP1 are not found in the other Cd-thioneins. When Cd-BP1 is acidified (pH 2.0) and successively neutralized, a shoulder of 265 nm in the UV spectrum and a Cotton band at 275 nm disappear, and the molecular weight changes from 4000 to 1800, with simultaneous loss of the labile sulfide. While the reconstituted complex without labile sulfide showed the characteristics of Cd-BP2, the reconstituted complex in the presence of labile sulfide indicated partial reconstitution of Cd-BP1. The UV and CD spectra differences between reconstituted and native Cd-BP1 suggest the requirement for some additional molecular architecture including another peptide-Cd2+ interaction. Induction of cadystin synthesis is almost exclusive for Cd, but an exception is a small amount of cadystin also induced by the higher concentration of CuCl2 (2.5 mM).(ABSTRACT TRUNCATED AT 250 WORDS)