THE DNA-BINDING AND ACTIVATION DOMAINS OF GAL4P ARE SUFFICIENT FOR CONVEYING ITS REGULATORY SIGNALS

The transcriptional activation function of the Saccharomyces cerevisia e activator Gal4p is known to rely on a DNA binding activity at its am ino terminus and an activation domain at its carboxy terminus, Althoug h both domains are required for activation, truncated forms of Gal4p c ontaining only these domains activate poorly in vivo, Also, mutations in an internal conserved region of Gal4p inactivate the protein, sugge sting that this internal region has some function critical to the acti vity of Gal4p. We have addressed the question of what is the minimal f orm of Gal4 protein that can perform all of its known functions, A for m with an internal deletion of the internal conserved domain of Gal4p is transcriptionally inactive, allowing selection for suppressors. All suppressors isolated were intragenic alterations that had further ami no acid deletions (miniGAL4s). Characterization of the most active min iGal4 proteins demonstrated that they possess all of the known functio ns of full-length Gal4p, including glucose repression, galactose induc tion, response to deletions of gal11 or gal6, and interactions with ot her proteins such as Gal80p, Sug1p, and TATA binding protein, Analysis of the transcriptional activities, protein levels, and DNA binding ab ilities of these miniGal4ps and a series of defined internal mutants c ompared to those of the full-length Gal4p indicates that the DNA bindi ng and activation domains are necessary and sufficient qualitatively f or all of these known functions of Gal4p. Our observations imply that the internal region of Gal4 protein may serve as a spacer to augment t ranscription and/or may be involved in intramolecular or Gal4p-Gal4p i nteractions.