Genetic evidence that an activation domain of GAL4 does not require acidity and may form a beta sheet.

Regulation of gene expression in eukaryotes relies on intricate protein-protein interactions. Transcription of the galactose genes in yeast has been a productive model for this type of interaction. The positive activator in this system, GAL4, has a bifunctional C-terminus. It contains both a prototypic acidic activation domain and a region that binds the negative regulator, GAL80. We have taken advantage of this colocalization of functions to subject the region to a constrained mutagenesis analysis: one function was maintained, while the other one was altered. This analysis and the experiments it suggested have led us to two conclusions: first, the acidic amino acids are not, as commonly thought, required for activation; second, this region is not unstructured or alpha helical, but its function may require a beta sheet.