Metabolic plasticity of wild-type and AOX-deficient Chlamydomonas reinhardtii cells related to the inorganic nitrogen source (nitrate or ammonium), as revealed by a 2D-DIGE comparative proteomic analysis

In the model unicellular green alga Chlamydomonas reinhardtii, both nitrate and ammonium can be used as primary inorganic nitrogen sources. Interestingly, the expression of the mitochondrial alternative oxidase (AOX), an "energy-dissipating" ubiquinol-oxygen oxidoreductase of the mitochondrial electron transport chain, is under the control of the exogenous nitrogen source : it is activated in nitrate-grown cells and repressed in ammonium-grown cells at both transcriptional and translational levels. This regulation of AOX by nitrogen is Chlamydomonas-specific and currently its bioenergetic and metabolic significance is poorly understood. In order to get clues to this peculiar phenomenon, we characterized the global metabolic response of a wild-type strain (WT) and an AOX-deficient mutant (AOX-) obtained by RNA interference grown either on nitrate or ammonium. For this purpose, we used a highly accurate 2D electrophoresis-based comparative proteomic approach (2D-DIGE) to compare the cellular proteomes of nitrate and ammonium-grown WT and AOX- Chlamydomonas. The analysis was performed in the middle of the exponential growth phase in mixotrophic conditions. It revealed many proteomic modifications between WT and AOX- cells and a smaller number between nitrate and ammonium-grown cells. In nitrate-grown cells, we notably observed an important up-regulation of glutamine synthetase. Interestingly, in AOX- cells, we respectively detected a general down-regulation and a general up-regulation of mitochondrial and chloroplastic bioenergetic enzymes, and also an important up-regulation of glutathione-dependent oxidative stress defense systems together with a remarkable down-regulation of methionine synthase. Altogether these results and previous studies provide new features in understanding the metabolic adaptations occurring in response to the inorganic nitrogen source with emphasis on the role played by AOX.