AUA1, A GENE INVOLVED IN AMMONIA REGULATION OF AMINO-ACID-TRANSPORT IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae the general amino acid (GAP1) permease catalyses active transport of apparently all amino acids across the plasma membrane. GAP1 activity is regulated by control of synthesis and control of activity in response to the nitrogen source supplied; ammonia and glutamine inactivate GAP1 function while proline and urea allow its maximum expression. We have isolated and characterized a gene, AUA1, involved in ammonia regulation of GAP1 activity. AUA1 is not essential for growth but overexpression of the AUA1 transcript in a high-copy vector or due to a regulatory mutation, aua1-1, present approximately 10 bp upstream from the start of AUA1 transcription, releases GAP1 activity from ammonia-inactivation without affecting GAP1 transcription. The aua1-1 mutation has no phenotype when ammonia is replaced by proline or glutamate as the nitrogen source or when it is present in a gap1 background. AUA1 expression is itself ammonia repressible in a wild-type strain but not in the aua1-1 mutant. The AUA1 gene sequence contains a unique short open reading frame of 94 codons corresponding to a polypeptide of 11 714 Da. This polypeptide is highly hydrophilic and extremely basic. The AUA1 product shows no significant similarity with any previously known protein sequence. Interestingly, a 10-amino acid segment of AUA1 is directly repeated in the most basic segment of the protein. Possible roles of AUA1 are discussed.