THE MOLECULAR-GENETICS OF NITROGEN ASSIMILATION INTO AMINO-ACIDS IN HIGHER-PLANTS

Nitrogen assimilation is a vital process controlling plant growth and development. Inorganic nitrogen is assimilated into the amino acids gl utamine, glutamate, asparagine, and aspartate, which serve as importan t nitrogen carriers in plants. The enzymes glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), aspartate a minotransferase (AspAT), and asparagine synthetase (AS) are responsibl e for the biosynthesis of these nitrogen-carrying amino acids. Biochem ical studies have revealed the existence of multiple isoenzymes for ea ch of these enzymes. Recent molecular analyses demonstrate that each e nzyme is encoded by a gene family wherein individual members encode di stinct isoenzymes that are differentially regulated by environmental s timuli, metabolic control, developmental control, and tissue/cell-type specificity. We review the recent progress in using molecular-genetic approaches to delineate the regulatory mechanisms controlling nitroge n assimilation into amino acids and to define the physiological role o f each isoenzyme involved in this metabolic pathway.