Bleomycin (Bm) in the culture broth of Streptomyces verticillus is complexed with Cu(2+) (Cu(II)). In the present study, we determined the x-ray crystal structures of the Cu(II)-bound and the metal-free types of Bm at a high resolution of 1.6 and 1.8 A, respectively, which are complexed with a Bm resistance determinant from Bm-producing S. verticillus, designated BLMA. In the current model of Cu(II).Bm complexed with BLMA, two Cu(II).Bm molecules bind to the BLMA dimer. The electron density map shows that the copper ion is clearly defined in the metal-binding domain of the Bm molecule. The metal ion is penta-coordinated by a tetragonal monopyramidal cage of nitrogens and binds to the primary amine of the beta-aminoalanine moiety of Bm. The binding experiment between Bm and BLMA showed that each of the two Bm-binding pockets has a different dissociation constant (K(d)(1) and K(d)(2)). The K(d)(1) value of 630 nm for the first Bm binding is larger than the K(d)(2) value of 120 nm, indicating that the first Bm binding gives rise to a cooperative binding of the second Bm to the other pocket.