Solution Structure of the DNA-binding Domain of GAL4 from Saccharomyces cerevisiae

The GAL4 transcript ional activator protein has long been a favorite for the study of transcription in eukaryotic biology. Genetic studies reveal a modular architecture for the protein with different functions associated with each module [1]. A DNA-binding domain of the protein recognizes and binds to a sequence of DNA termed the Upstream Activating Sequence ( U A S Q ) . Other parts of the protein are relevant for activation. They interact with the transcriptional machinery including RNA polymerase to activate transcription. The U A S Q is near the genes that encode the proteins required for galactose utilization. Upon presentation of galactose to the yeast cell, this DNA site is specifically bound by GAL4, the transcription function of RNA polymerase is activated, and enzymes required for galactose utilization are produced [2]. As a dimer of 881 amino acids, GAL4 is too large for the determination of a high-resolution NMR structure and we have instead studied a fragment containing the Nterminal 65 amino acid residues including its DNA-binding domain [3]. In the absence of DNA, GAL4(65) is monomeric in solut ion, presumably because it does not have the amino acid residues necessary for dimerization [4]. GAL4(65) is dimeric when bound to any of four DNA sites present in the UAS [5]. Each of these binding sites is approximately twofold palindromic, and like many other dimeric DNA-binding proteins, each monomer of the GAL4 dimer interacts with a half-site of DNA. The structure and dynamics of the monomeric DNA-binding domain of GAL4 (residues 1-65; Figure 1), an investigation of the binding to a DNA half-site (Figure 2), and the structure of the resultant protein-DNA complex are presented in this paper.