AMINO-ACIDS INDUCE EXPRESSION OF BAP2, A BRANCHED-CHAIN AMINO-ACID PERMEASE GENE IN SACCHAROMYCES-CEREVISIAE

Branched-chain amino acid uptake in Saccharomyces cerevisiae is mediat ed by at least three transport systems: the general amino acid permeas e Gap1p, the branched-chain amino acid permease Bap2p, and one or more so far unknown permeases. Regulation of the transcription of BAP2 is mainly subject to the presence of certain amino acids in the medium. T he level of transcription is low during growth on a minimal medium wit h proline as the sole nitrogen source. As assayed with a lacZ fusion, the level of transcription is slightly higher (3-fold) on a minimal me dium with ammonium ions as a nitrogen source, and transcription is ind uced about 20-fold by addition of leucine (0.2 mM). As little as 10 mu M leucine causes a fivefold induction, Addition of L-leucine to minim al proline medium, on the other hand, has no effect on BAP2 transcript ion. The two known permeases for transport of branched-chain amino aci ds, Gap1p and Bap2p, are thus not active at the same time. The BAP2 pr omoter contains one or two putative Gcn4p binding sites and one putati ve Leu3p binding site, None of the three is needed for induction by le ucine. Induction of BAP2 transcription by leucine is accompanied by an increase in branched-chain amino acid uptake. This elevation is inter preted to be partly the result of an increased level of the Bap2p perm ease in the plasma membrane, because deletion of BAP2 slightly decreas es the induction of uptake. There is still a leucine-inducible increas e in branched-chain amino acid uptake in a Delta gap1 Delta bap2 strai n, indicating that BAP2 shares leucine induction with at least one rem aining branched-chain amino acid-transporting permease.