LEU3 of Saccharomyces cerevisiae encodes an 886-amino-acid polypeptide that activates transcription of at least five genes by binding to an upstream decanucleotide sequence. This activation is dependent on the inducer alpha-isopropylmalate, the synthesis of which is repressed by leucine. We created a 285-amino-acid LEU3 derivative by removing a large block of internal sequences, including a dense cluster of acidic residues. This deletion protein bound to the decanucleotide sequence in vitro and activated gene expression in vivo. In contrast to wild-type LEU3, the truncated LEU3 protein was an effective transcriptional activator when alpha-isopropylmalate synthesis was repressed by leucine.