Membrane potential independent transport of NH(3) in the absence of ammonium permeases in Saccharomyces cerevisiae

BACKGROUND: Microbial production of nitrogen containing compounds requires a high uptake flux and assimilation of the N-source (commonly ammonium), which is generally coupled with ATP consumption and negatively influences the product yield. In the industrial workhorse Saccharomyces cerevisiae, ammonium (NH(4) (+)) uptake is facilitated by ammonium permeases (Mep1, Mep2 and Mep3), which transport the NH(4) (+) ion, resulting in ATP expenditure to maintain the intracellular charge balance and pH by proton export using the plasma membrane-bound H(+)-ATPase. RESULTS: To decrease the ATP costs for nitrogen assimilation, the Mep genes were removed, resulting in a strain unable to uptake the NH(4) (+) ion. Subsequent analysis revealed that growth of this ∆mep strain was dependent on the extracellular NH(3) concentrations. Metabolomic analysis revealed a significantly higher intracellular NH(X) concentration (3.3-fold) in the ∆mep strain than in the reference strain. Further proteomic analysis revealed significant up-regulation of vacuolar proteases and genes involved in various stress responses. CONCLUSIONS: Our results suggest that the uncharged species, NH(3), is able to diffuse into the cell. The measured intracellular/extracellular NH(X) ratios under aerobic nitrogen-limiting conditions were consistent with this hypothesis when NH(x) compartmentalization was considered. On the other hand, proteomic analysis indicated a more pronounced N-starvation stress response in the ∆mep strain than in the reference strain, which suggests that the lower biomass yield of the ∆mep strain was related to higher turnover rates of biomass components. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12918-016-0381-1) contains supplementary material, which is available to authorized users.