REGULATION OF EXPRESSION OF THE AMINO-ACID TRANSPORTER GENE BAP3 IN SACCHAROMYCES-CEREVISIAE

The BAP3 gene of Saccharomyces cerevisiae encodes a protein with a hig h similarity to the BAP2 gene product, a high-affinity permease for br anched-chain amino acids. In this paper, we show that, like BAP2, the expression of the BAP3 gene in S. cerevisiae is induced by the additio n of branched-chain amino acids to the medium. Unexpectedly, most othe r naturally occurring L-amino acids found in proteins (with the except ion of proline, lysine, arginine and histidine) have the same effect o n the expression of BAP3, The induction of BAP3 expression appears to be dependent on Stp1p, a nuclear protein, previously shown to be invol ved in pre-tRNA maturation and also required for the expression of BAP 2, as induction is no longer observed in an stp1(-) mutant. The transc riptional regulator Leu3p is not involved in the induction of BAP3 exp ression, but may act as a repressor of BAP3 expression in the absence of leucine, as can be inferred from a transcriptional analysis in a De lta leu3 mutant, By extensive deletion analysis of the BAPS promoter f used to a GUS reporter, as well as by fusions of different parts of th e BAP3 promoter to a LacZ reporter, we have found that a portion of th e BAPS promoter from -418 to -392 relative to the ATG start codon is b oth necessary and sufficient for the Stp1p-dependent induction of BAPS expression by (most) amino acids. We have therefore named this sequen ce UAS(aa) (amino acid-dependent upstream activator sequence), Neither Stp1p nor Leu3p appear to bind to the UAS(aa), at least in vitro, as judged from gel retardation assays, Sequences similar to the UAS(aa) c an be found in the promoters of BAP2, PTR2 and TAT1; genes that, like BAP3, encode permeases inducible by amino acids, suggesting that amino acid induction of all these genes is exerted via a common mechanism.