Fig. 5: The GS-GOGAT pathway assimilates ammonium to synthesize glutamate under nitrogen starvation.

a Schematic of ammonium assimilation in S. cerevisiae. GDH: glutamate dehydrogenase (Gdh1p and Gdh3p); GS-GOGAT: glutamine synthetase (Gln1p)-glutamine oxoglutarate aminotransferase (Glt1p); α-KG: α-ketoglutarate. There are two major routes for glucose-derived carbon to enter the TCA cycle: acetyl-CoA to citrate (catalyzed by Cit1p and Cit3p) and pyruvate to oxaloacetate (catalyzed by Pyc1p and Pyc2p). b Schematic of the experimental design for c, d, and e. Cells were grown in glutamate-fortified SD medium and shifted to [U-¹³C]-SD-N for 2 h. c Loss of ammonium assimilation impairs glutamate synthesis under nitrogen starvation. Three independent replicates. d Glutamate synthesis is impeded in glt1Δ cells, but not in gdh1Δgdh3Δ cells under nitrogen starvation. Three independent replicates. e The GS-GOGAT pathway predominantly assimilates ammonium to support glutamate synthesis under nitrogen starvation. Three independent replicates. f Schematic of experimental design for g. Cells grown in SD were labeled with ¹⁵N for varying times and then starved in [U-¹³C]-SD-N for 2 h. g Autophagy degrades macromolecules (e.g., proteins) to provide nitrogen for glutamate and aspartate synthesis under nitrogen starvation. Three independent replicates. P values were calculated using unpaired two-sided Student’s t test assuming equal variances. Data are presented as mean ± standard deviation.