OPTICAL-ACTIVITY ASSOCIATED WITH THE SULFUR TO METAL CHARGE-TRANSFER BANDS OF ZN AND CD GAL4

The N-terminal DNA-binding domain of the GAL4 transcription factor, isolated as a proteolytic fragment of 63 amino acid residues, GAL4(63), or cloned as a 62-residue subdomain, GAL4(62*), binds 2 Cd(II) ions. Both Cd derivatives show intense ellipticity bands associated with -S- --> Cd charge transfer bands at 250 nm (+), 233 nm (-), and 218 nm (+). Molar ellipticity values are (5-7) x 10(4) deg.cm2.dmol-1. The energies and pattern (+, -, +) of these ellipticity bands are similar to those observed for the Cd(II) clusters of metallothionein. The latter are believed to be due to the presence of bridging cysteine ligands (Willner, H.W., Vasak, M., & Kagi, J.H.R., 1987, Biochemistry 26, 6287-6292). Circular dichroism provides further evidence that the GAL4 DNA-binding domain forms a binuclear Cd cluster with the amino acid sequence -C-11-X2-C-14-X6-C21-X6-C28-X2-C31-X6-C38- found in GAL4(62*). Zn2GAL4(62*) has much weaker ellipticity associated with the higher energy -S- --> Zn charge transfer bands. Removal of the metal ions from GAL4(62*) results in substantial changes in the near UV circular dichroism due to both loss of the charge transfer bands and changes in the conformation of the peptide backbone. Binding of the UAS(G) enhancer DNA element to either Zn or Cd GAL4(62*) alters the ellipticity of the -S- --> metal charge transfer bands suggesting that DNA binding is associated with some changes in conformation around the metal cluster.