The SSY1 gene of Saccharomyces cerevisiae encodes a member of a large family of amino acid permeases. Compared to the 17 other proteins of this family, however, Ssy1p displays unusual structural features reminiscent of those distinguishing the Snf3p and Rgt2p glucose sensors from the other proteins of the sugar transporter family. We show here that SSY1 is required for transcriptional induction, in response to multiple amino acids, of the AGP1 gene encoding a low-affinity, broad-specificity amino acid permease. Total noninduction of the AGP1 gene in the ssy1⌬ mutant is not due to impaired incorporation of inducing amino acids. Conversely, AGP1 is strongly induced by tryptophan in a mutant strain largely deficient in tryptophan uptake, but it remains unexpressed in a mutant that accumulates high levels of tryptophan endogenously. Induction of AGP1 requires Uga35p(Dal81p/DurLp), a transcription factor of the Cys 6 -Zn 2 family previously shown to participate in several nitrogen induction pathways. Induction of AGP1 by amino acids also requires Grr1p, the F-box protein of the SCF Grr1 ubiquitin-protein ligase complex also required for transduction of the glucose signal generated by the Snf3p and Rgt2p glucose sensors. Systematic analysis of amino acid permease genes showed that Ssy1p is involved in transcriptional induction of at least five genes in addition to AGP1. Our results show that the amino acid permease homologue Ssy1p is a sensor of external amino acids, coupling availability of amino acids to transcriptional events. The essential role of Grr1p in this amino acid signaling pathway lends further support to the hypothesis that this protein participates in integrating nutrient availability with the cell cycle.

In Saccharomyces cerevisiae, the transport of ammonium across the plasma membrane for use as a nitrogen source is mediated by at least two functionally distinct transport systems whose respective encoding genes are called MEP1 and MEP2. Mutations in the MEP2 gene affect high affinity, low capacity ammonium transport while mutations in the MEP1 gene disrupt a lower affinity, higher capacity system. In this work, the MEP1 gene has been cloned and sequenced and its expression analyzed. The predicted amino acid sequence reveals a highly hydrophobic, 54 kDa protein with 10 or 11 putative membrane‐spanning regions. The predicted Mep1p protein shares high sequence similarity with several bacterial proteins of unknown function, notably the product of the nitrogen‐regulated nrgA gene of Bacillus subtilis, and with that of a partial cDNA sequence derived from Caenorhabditis elegans. The Mep1p and related proteins appear to define a new family of transmembrane proteins evolutionarily conserved in at least bacteria, fungi and animals. The MEP1 gene is most highly expressed when the cells are grown on low concentrations of ammonium or on ‘poor’ nitrogen sources like urea or proline. It is down‐regulated, on the other hand, when the concentration of ammonium is high or when other ‘good’ nitrogen sources like glutamine or asparagine are supplied in the culture medium. The overall properties of Mep1p indicate that it is a transporter of ammonium. Its main function appears to be to enable cells grown under nitrogen‐limiting conditions to incorporate ammonium present at relatively low concentrations in the growth medium.

We compared the transcriptomes of Saccharomyces cerevisiae cells growing under steady-state conditions on 21 unique sources of nitrogen. We found 506 genes differentially regulated by nitrogen and estimated the activation degrees of all identified nitrogen-responding transcriptional controls according to the nitrogen source. One main group of nitrogenous compounds supports fast growth and a highly active nitrogen catabolite repression (NCR) control. Catabolism of these compounds typically yields carbon derivatives directly assimilable by a cell's metabolism. Another group of nitrogen compounds supports slower growth, is associated with excretion by cells of nonmetabolizable carbon compounds such as fusel oils, and is characterized by activation of the general control of amino acid biosynthesis (GAAC). Furthermore, NCR and GAAC appear interlinked, since expression of the GCN4 gene encoding the transcription factor that mediates GAAC is subject to NCR. We also observed that several transcriptional-regulation systems are active under a wider range of nitrogen supply conditions than anticipated. Other transcriptional-regulation systems acting on genes not involved in nitrogen metabolism, e.g., the pleiotropic-drug resistance and the unfolded-protein response systems, also respond to nitrogen. We have completed the lists of target genes of several nitrogen-sensitive regulons and have used sequence comparison tools to propose functions for about 20 orphan genes. Similar studies conducted for other nutrients should provide a more complete view of alternative metabolic pathways in yeast and contribute to the attribution of functions to many other orphan genes.

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Aromatic aminotransferase II, product of the ARO9 gene, catalyzes the first step of tryptophan, phenylalanine, and tyrosine catabolism in Saccharomyces cerevisiae. ARO9 expression is under the dual control of specific induction and nitrogen source regulation. We have here identified UAS aro , a 36-bp upstream element necessary and sufficient to promote transcriptional induction of reporter gene expression in response to tryptophan, phenylalanine, or tyrosine. We then isolated mutants in which UAS aro -mediated ARO9 transcription is partially or totally impaired. Mutations abolishing ARO9 induction affect a gene called ARO80 (YDR421w), coding for a Zn 2 Cys 6 family transcription factor. A sequence highly similar to UAS aro was found upstream from the YDR380w gene encoding a homolog of bacterial indolepyruvate decarboxylase. In yeast, this enzyme is postulated to catalyze the second step of tryptophan catabolism to tryptophol. We show that ARO9 and YDR380w (named ARO10) have similar patterns of transcriptional regulation and are both under the positive control of Aro80p. Nitrogen regulation of ARO9 expression seems not directly to involve the general factor Ure2p, Gln3p, Nil1p, Uga43p, or Gzf3p. ARO9 expression appears, rather, to be mainly regulated by inducer exclusion. Finally, we show that Gap1p, the general amino acid permease, and Wap1p (Ycl025p), a newly discovered inducible amino acid permease with broad specificity, are the main aromatic amino acid transporters for catabolic purposes.The yeast Saccharomyces cerevisiae can use tryptophan, phenylalanine, or tyrosine as the only source of cellular nitrogen (11). The main products of their catabolism are tryptophol, phenylethanol, and tyrosol, respectively, constituents of the mixture of alcohols collectively known as fusel oil in fermentations (41,42,66,69). Fusel oil formation from amino acids is believed to proceed through the so-called Ehrlich pathway involving three enzymatic steps. A first transamination produces the ␣-keto-acid analog of the amino acid, a decarboxylation step yields an aldehyde, and a reduction step converts the aldehyde to a primary alcohol (79). Tryptophan is thus converted to tryptophol via the metabolic intermediates indole-3-pyruvate and indole-3-aldehyde (41, 69); tyrosine and phenylalanine are converted to tyrosol and phenylethanol in a similar way (42,66). Yet as recently stressed for leucine catabolism by Dickinson et al. (15), this general scheme of amino acid degradation stems mainly from studies of metabolic intermediates and end product formation. Too little is known about the specific permeases and enzymes involved in aromatic amino acid utilization, their genetic determinants, and their regulation. Aromatic aminotransferase II, the product of the recently cloned ARO9 gene (34), is the first characterized enzyme proposed to be involved in the catabolism of aromatic amino acids. Specifically, (i) ARO9 gene transcription is induced by the presence of tryptophan, phenylalanine, or tyrosine in the growth medium and remains a...