Fig. 6: Aspartate is a major nitrogen investment under nitrogen starvation.

a Proposed model for glutamate and aspartate metabolism under nitrogen starvation. Only ammonium-independent glutamate synthesis is shown. The large font for Aat2p indicates that it acts as the major enzyme for aspartate synthesis under nitrogen starvation. b Loss of aspartate synthesis leads to strong accumulation of newly synthesized glutamate. Cells were grown in SD + 10 mM aspartate (pH 5.4) and starved in [U-¹³C]-SD-N for 2 h. Two independent replicates for aat2Δatg1Δ (aspartate), five for aat1Δaat2Δatg1Δ, and three for the remaining strains. c Loss of phenylalanine and tyrosine synthesis does not significantly affect newly synthesized glutamate under nitrogen starvation. Cells were grown in SD + phenylalanine, tyrosine, and tryptophan at 1 mM each and shifted to [U-¹³C]-SD-N for 2 h. The aro8Δaro9Δ mutant is not auxotrophic for tryptophan, as evidenced by the detection of [U-¹³C]-tryptophan under nitrogen starvation. Nonetheless, we supplemented tryptophan to compare it with phenylalanine and tyrosine as an amino-group donor for glutamate synthesis under nitrogen starvation. Three independent replicates. d Loss of valine and isoleucine synthesis led to modest accumulation of newly synthesized glutamate under nitrogen starvation. Strains were grown in SD + isoleucine, leucine, and valine at 1 mM each and shifted to [U-¹³C]-SD-N for 2 h. Three independent replicates. The presence of [U-¹³C]-leucine in the bat1Δbat2Δ mutant indicates the existence of additional enzyme(s) for leucine synthesis. Data are presented as mean ± standard deviation.