Expression of the BAP3 gene of Saccharomyces cerevisiae, encoding a branched chain amino acid permease, is induced in response to the availability of several naturally occurring amino acids in the medium. This induction is mediated via an upstream activating sequence (called UAS(aa)) in the BAP3 promoter, and dependent on Stp1p, a nuclear protein with zinc finger domains, suggesting that Stp1p is a transcription factor involved in BAP3 expression. In this paper, we show that Stp2p, a protein with considerable similarity to Stp1p, is also involved in the induction of BAP3 expression. To gain more insight into the roles of STP1 and STP2, we have overexpressed both Stp1p and Stp2p in yeast cells. Gel shift assays with the UAS(aa)as a probe show that the UAS(aa)can form two major complexes. One complex is dependent on Stp2p overexpression and the other is formed independently of STP1 or STP2, suggesting that the UAS(aa)is also bound by another factor. Here we show that the other factor is Abf1p, which binds specifically to the UAS(aa)of BAP3.