The transcriptional activation function of the Saccharomyces cerevisiae activator Gal4p is known to rely on a DNA binding activity at its amino terminus and an activation domain at its carboxy terminus. Although both domains are required for activation, truncated forms of Gal4p containing only these domains activate poorly in vivo. Also, mutations in an internal conserved region of Gal4p inactivate the protein, suggesting that this internal region has some function critical to the activity of Gal4p. We have addressed the question of what is the minimal form of Gal4 protein that can perform all of its known functions. A form 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 amino acid deletions (miniGAL4s). Characterization of the most active miniGal4 proteins demonstrated that they possess all of the known functions of full-length Gal4p, including glucose repression, galactose induction, response to deletions of gal11 or gal6, and interactions with other proteins such as Ga180p, Sug1p, and TATA binding protein. Analysis of the transcriptional activities, protein levels, and DNA binding abilities of these miniGal4ps and a series of defined internal mutants compared to those of the full-length Gal4p indicates that the DNA binding and activation domains are necessary and sufficient qualitatively for all of these known functions of Gal4p. Our observations imply that the internal region of Gal4 protein may serve as a spacer to augment transcription and/or may be involved in intramolecular or Gal4p-Gal4p interactions.