Interaction of cytosolic and plastidic nitrogen metabolism in plants.

In angiosperms, the assimilation of ammonia resulting from nitrate reduction and from photorespiration depends on the operation of the plastidic GS/GOGAT cycle. The precursor for ammonia assimilation, 2-oxoglutarate, is synthesized in the mitochondria and in the cytosol. It is imported into the plastid by a 2-oxoglutarate/malate translocator (DiT1). In turn, the product of ammonia assimilation, glutamate, is exported from the plastids by a glutamate/malate translocator (DiT2). These transport processes link plastidic and cytosolic nitrogen metabolism and are essential for plant metabolism. DiT1 was purified to homogeneity from spinach chloroplast envelope membranes and identified as a protein with an apparent molecular mass of 45 kDa. Peptide sequences were obtained from the protein and the corresponding cDNA was cloned. The function of the DiT1 protein and its substrate specificity were confirmed by expression of the cDNA in yeast cells and functional reconstitution of the recombinant protein into liposomes. Recent advances in the molecular cloning of DiT2 and in the analysis of the in vivo function of DiT1 by antisense repression in transgenic tobacco plants will be discussed. In non-green tissues, the reducing equivalents required for glutamate formation by NADH-GOGAT are supplied by the oxidative pentose phosphate pathway. Glucose 6-phosphate, the immediate precursor of the oxidative pentose phosphate pathway is generated in the cytosol and imported into the plastids by the plastidic glucose 6-phosphate/phosphate translocator.