Transcriptional regulation of the Saccharomyces cerevisiae amino acid permease gene BAP2

Uptake of branched-chain amino acids by Saccharomyces cerevisiae from media containing a preferred nitrogen source is mediated by the permeases encode d by BAP2, BAP3, and VAP1/TAT1. The transcriptional activity of the BAP2 pr omoter is affected by a number of genes, including SSY1, which encodes an a mino acid permease homologue that is necessary for transcription of BAP2. O ther genes that control BAP2 encode known (Leu3p, Tup1p) and putative (Stp1 p, Stp2p) transcription factors. We present evidence that the zinc-finger p roteins Stp1p and Stp2p bind directly to the BAP2 promoter. Binding of Stp1 p to the BAP2 promoter in vivo and in vitro indicates that the STP gene fam ily indeed encodes transcription factors. The presence of a Leu3p binding s ite in the BAP2 promoter is required for full promoter activity on syntheti c complete medium. The capacity of Leu3p to activate BAP2 transcription cor relates with conditions that affect the level of alpha -isopropyl malate. T he effect of a tup1 deletion on BAP2 transcription depends on SSY1. In an s sy1 strain, the phenotype of tup1 conforms to the well-established role of Tup1p as part of a repressor complex, but in the SSY1 strain deletion of TU P1 causes a decrease in transcription, indicating that Tup1p may also have an activating role at the BAP2 promoter. Our results thus suggest a complex interplay between several transcription factors in the expression of BAP2.