GLN3 encodes a transcriptional activator that is involved in positively regulating genes that are subject to nitrogen catabolite repression (NCR) (3). Gln3p, via a zinc finger binding domain (4), binds to the sequence 5-GATAAG-3 in the UAS_NTR found in the promoters of many genes involved in nitrogen utilization (5, 6). When nitrogen is limiting, Gln3p is localized to the nucleus and the expression of NCR-sensitive genes is upregulated (7). In contrast, when optimal nitrogen sources are available, Gln3p is localized to the cytoplasm and transcription of NCR-sensitive genes is minimal (7). Nuclear import and export of Gln3p depends on the proteins Srp1p and Crm1p respectively (8). The Ran GTPase pathway and an intact actin cytoskeleton and are also required for proper Gln3p nuclear translocation (8, 9).

Gln3p is a phosphoprotein, and phosphorylation appears to be dependent on the quality of the nitrogen source in the media. When cells are provided with good nitrogen sources, Gln3p is phosphorylated by the TOR kinases Tor1p and Tor2p (7, 10). While the TOR kinases are the primary mediators of Gln3p phosphorylation, there is evidence that the kinases Snf1p and Npr1p may also be involved in Gln3p regulation (8, 11, 12). The phosphorylated form of the Gln3p is bound by the regulatory protein Ure2p and sequestered in the cytosol (13, 14). The Gln3p:Ure2p complex is significantly more resistant to dephosphorylation than free Gln3p (10). Gln3p dephosphorylation and subsequent activation can be induced by: nitrogen limitation (7), glutamine starvation (15), treatment with the TOR inhibitor drug rapamycin (7), regulation by the phosphatases Pph3p or Sit4p (7, 10), or the loss of the regulatory subunit of Sit4p, Tap42p (7, 10). While most studies indicate it is the dephosphorylated form of Gln3p that is active, treatment of cells with the glutamine synthetase inhibitor methionine sulfoximine increases the phosphorylation state of Gln3p but results in nuclear localization and activation of the protein (16). In addition to phosphorylation, protein stability might also be an important aspect of Gln3p regulation. Under poor nitrogen conditions, Gln3p activation is dependent on the E3/E4 ubiquitin-protein ligase protein Rsp5p and the Rsp5-associated proteins Bul1p and Bul2p (12).

Although an increase in copies of GLN3 causes drastic decreases in the cellular growth rate, GLN3 is not essential for growth (4). Loss of Gln3p also confers resistance to the drug rapamycin (17), and for some genes the absence of Gln3p results in a longer mRNA transcript because the GATAAG binding sites can serve as a surrogate TATA transcription initiation site (13).

Last updated: 2005-09-29