In S. cerevisiae cells, the amino group of glutamate and the amide group of glutamine are the source of nitrogen for biosynthesis of all other macromolecules (1). In order to provide ammonia for the synthesis of glutamine during growth on glutamate-yielding nitrogen sources, cells degrade glutamate into ammonia and oxaloacetate. The main pathway for S cerevisiae glutamate degradation, shown here, is catalyzed by the NAD-dependent glutamate dehydrogenase encoded by GDH2 (1).

Transcriptional regulation of GDH2 is dependent upon six different sequence elements present in the promoter (5). Two elements behave as upstream activation sites (UAS) while the remaining four elements inhibit the effects of the two UASs (5). These upstream repression site (URS) elements bear significant sequence similarity to the URS found in the CAR1 promoter, which is bound by the global repressor Ume6p (5, 6). One UAS has been shown to activate transcription during growth on nonfermentable carbon sources or on limiting amounts of glucose (7). The other UAS, UAS_NTR, is under the control of nitrogen catabolite repression (NCR) (8). Under nitrogen-poor conditions, GDH2 expression is upregulated by the transcriptional activator Gln3p, which binds to the UAS_NTR, and the co-activator Hfi1p, which links Gln3p to the Ada2/Gcn5/Ada3 transcriptional activator complex (9, 10). Conversely, in the presence of an optimal nitrogen source such as glutamine, GDH2 transcription is repressed by sequestration of Gln3p to the cytosol by the transcription factor Ure2p (11). Unrelated to NCR, GDH2 gene expression is regulated by ammonia concentration. An increase in intracellular ammonia leads to a decrease in GDH2 expression (3) but an increase in extracellular ammonia leads to an increase in both GDH2 transcription and Gdh2p activity (12). Gdh2p enzyme activity also appears to be regulated by phosphorylation through cAMP-dependent and cAMP-independent protein kinases and subsequent proteolysis (13, 14).

Loss of Gdh2p leads to impaired glutamate utilization and severely reduced growth on poor nitrogen sources (1). Although Gdh2p is not normally involved in glutamate biosynthesis, overexpression of GDH2 can promote catalysis of the reverse reaction (1). Overexpression of GDH2 also suppresses the phenotype of cells lacking the phosphoglucose isomerase Pgi1p, which cannot grow on glucose as the sole carbon source (15). Deficiencies in the human homologs of glutamate dehydrogenase, GLUD1 (OMIM) and GLUD2 (OMIM), have been linked to hyperinsulinism-hyperammonemia syndrome (OMIM) (16, 17) and various neurological disorders (18).

Last updated: 2005-10-11