Transcriptional regulation of the Saccharomyces cerevisiae amino acid permease gene BAP2

Abstract.

Uptake of branched-chain amino acids by Saccharomyces cerevisiae from media containing a preferred nitrogen source is mediated by the permeases encoded by BAP2, BAP3, and VAP1/TAT1. The transcriptional activity of the BAP2 promoter is affected by a number of genes, including SSY1, which encodes an amino acid permease homologue that is necessary for transcription of BAP2. Other genes that control BAP2 encode known (Leu3p, Tup1p) and putative (Stp1p, Stp2p) transcription factors. We present evidence that the zinc-finger proteins Stp1p and Stp2p bind directly to the BAP2 promoter. Binding of Stp1p to the BAP2 promoter in vivo and in vitro indicates that the STP gene family indeed encodes transcription factors. The presence of a Leu3p binding site in the BAP2 promoter is required for full promoter activity on synthetic complete medium. The capacity of Leu3p to activate BAP2 transcription correlates with conditions that affect the level of α-isopropyl malate. The effect of a tup1 deletion on BAP2 transcription depends on SSY1. In an ssy1 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 TUP1 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.