Structure of the DNA-binding domain of zinc GAL4

THE yeast transcriptional activator GAL4 binds co-operatively to four related 17-base-pair sequences within an upstream activating sequence (UASG) to activate transcription of the GAL1 and GAL10 genes1. It belongs to a class of gene regulatory proteins which all contain a highly conserved cysteine-rich region within their DNA-binding domains2,3. This region binds zinc4–7 and it has been proposed that the cysteine residues coordinate the zinc, creating a structure analogous to one of the 'zinc fingers' of the transcription factor TFHIA (ref. 8). Using 1H–113Cd two-dimensional nuclear magnetic resonance spectra of the cadmium form of the domain, we previously showed that the protein contains a Cd2Cys6cluster where cysteines 11 and 28 act as bridging ligands9. A similar study of a fragment of GAL4 has recently been published10. We report here the solution structure of the DNA binding domain of GAL4; two helix-turn-strand motifs pack around a Zn2Cys6 cluster in a novel pseudo-symmetrical arrangement. The results show that the GAL4 zinc-binding domain differs significantly from both the TFIIIA-type zinc finger11 and the steroid hormone receptor DNA-binding domains12.