GAL4, DNA-binding domain
The nuclear protein GAL4 is a positive regulator of gene expression for the galactose-induced genes such as GAL1, GAL2, GAL7, GAL10, and MEL1. These genes encode enzymes which convert galactose to glucose. GAL4 recognizes a 17 base-pair long sequence in the upstream activating sequence (uas-g) of these genes, (5'-cggrnnrcynyncnccg-3'). GAL4 binds to the DNA as a homodimer, most likely forming a complex with GAL11.
This protein contains a fungal Zn(2)-Cys(6) binuclear cluster domain. Many transcriptional activator proteins possess such a domain in which six conserved cysteine residues bind to two zinc ions known as a binuclear zinc cluster. This cysteine-rich region binds to the DNA in a zinc-dependent fashion. While GAL4 from the organism Saccharomyces cerevisiae contains such a domain, it binds two Cadmium (Cd) ions rather than Zinc ions.
The metal-dependent DNA-binding domain of GAL4 is shown above. It is 43 amino acid residues long. It contains two right-handed alpha-helices shown here in red. The six conserved cysteine residues are shown in blue, and the two cadmium ions are shown in silver.