Role of phosphoenolpyruvate carboxylase in anaplerosis in the green microalga Dunaliella salina cultured under different nitrogen regimes.

Anaplerosis plays a very important role in providing C for N assimilation. In green algae and higher plants, phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) is the main anaplerotic carboxylase. On this basis we hypothesize that N availability affects PEPC expression. In order to test this hypothesis, the model organism Dunaliella salina was cultured under a variety of N growth regimes. Our results show that the level of PEC activity was unaffected by the N form in which N was supplied to the cells, when N concentration was low (0.5-0.01 mM). When cells were adapted to growth at 5 mM N, however, PEPC activity on a per cell basis was substantially higher in NH4+-adapted cells as compared to their NO3--adapted counterparts; however, the same difference was not observed on a protein basis. This notwithstanding, even at low N, PEPC of cells cultured in the presence of either NH4+ or NO3- appeared to differ in their molecular masses. These results suggest that cells adapted to different N-form express distinct PEPC isoforms. In addition to this, we observed that, in algae adapted to high (5 mM) NH4+ concentration, a PEPC isoform was induced that differed from the isoforms observed in algae adapted to lower concentrations of the same N-source. These findings lead us to conclude that the expression of PEPC isoforms in D. salina responds to the variation in the C-skeleton demand deriving from changes in the chemical form and availability of N.