Stp1p, Stp2p and Abf1p are involved in regulation of expression of the amino acid transporter gene BAP3 of Saccharomyces cerevisiae

Expression of the BAP3 gene of Saccharomyces cerevisiae, encoding a branche d chain amino acid permease, is induced in response to the availability of several naturally occurring amino acids in the medium. This induction is me diated via an upstream activating sequence (called UAS(aa)) in the BAP3 pro moter, 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 mo re insight into the roles of STP1 and STP2, we have overexpressed both Stp1 p and Stp2p in yeast cells. Gel shift assays with the UAS(aa) as a probe sh ow 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 ST P2, suggesting that the UAS(aa) is also bound by another factor. Here we sh ow that the other factor is Abf1p, which binds specifically to the UAS(aa) of BAP3.